• 用了几天,把刷过的Leetcode的题目总结了一下(不包括带锁的题目和最新的题目), 希望用一句或几句话把题目思路说清楚,还没有总结DP相关的内容,贴出来和大家分享一下,欢迎讨论,也为太阁会员抽奖赞赞人品,转载请注明出处。

    Math and Bit Manipulation

    1. Reverse Integer
      Do in one iteration, remember boundary Check
    2. Palindrome Integer
      Compare digit in head and tail
    3. Divide two Integer
      Binary Search
    4. Multiply String
      Do it as Math
    5. Pow(x,n)
      Fast Exponential, consider n might be negative
    6. Permutation Sequence
      Find the right bucket
    7. Plus one
    8. Add Binary
    9. Sqrt(x)
      Binary Search or (i = 0x5f3759df - ( i >> 1 ))
      10 Single Number
    10. Single Number II
    11. Fraction to recurring decimal
    12. Excel Sheet Column title
    13. Excel Sheet Column number
    14. Factorial Trailing zeros
      Count 5’s times
    15. Reverse bit
      Dividend and conquer and combine with bit operation
    16. Number of 1 bits
      while(n > 0) n = n & (n - 1); count ++
    17. Bitwise AND of numbers range
      Give m and n, return the result of m & m + 1 & m + 2 & m + 3 … & n
      Answer is Common prefix of two numbers
    18. Count prime
      Use Array to mark out non-prime number
    19. Rectangle area
      4 point defines two rectangle, calculate the rectangle area
      Sum of two rectangle - overlap of two rectangle
    20. Power of two
      n & (n - 1)
    21. Number of Digit one
      Count with bucket
    22. Add Digits
      Possible answer is 1 to 9. 1 + (num - 1) % 9
    23. Single Number 3
    24. Ugly Number 1
      Check if given number is ugly number or not
    25. Ugly Number 2
      Return nth ugly number, use heap(common heap or 3 heaps) to track the current smallest ugly number
    26. Missing Number
    27. Perfect square
      Given a number, return how many perfect square number can sum up to it
    28. Self crossing
      Divided to 3 scenario
    29. Power of 4
      Power of 2 + even number of trailing zero (num - 1 and count digit 1)
    30. Power of 3
      Bulb Switcher
    31. Count numbers with Unique Digits
      Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10n.
      32 Valid Perfect Square
      perfect square = 1 + 3 + 5 + 7 + ….
      33 Sum of two Integer
      XOR and carrier

    Add two numbers

    1. Remove Nth Node from End of List
      Fast/Slow pointer
    2. Merge Two Sorted List
    3. Merge K LinkedList
    4. Swap Nodes in Pairs
    5. Reverse Nodes in K-Group
    6. Rotate List
    7. Remove Duplicates from Sorted Linked List
      So that each element only appears once
    8. Remove Duplicates from Sorted Linked List
      So that all the dup elements are deleted
    9. Partition List
    10. Reverse Linked List from m to n
    11. Copy List with Random Pointer
      Double and separate
    12. LinkedList Cycle
      Fast/Slow pointer
    13. LinkedList Cycle II
    14. Reorder List
      L0->L1->…->Ln-1-> Ln to L0->Ln->L1->Ln-1->L2->Ln-2
      Halve the list, reverse the second half and merge two lists
    15. Intersection of Two Linked List
      Align the beginning parts
    16. Remove LinkedList Element
      Compare and remove, careful for the head might change
    17. Palindrome Linked List
      Find the mid element and reverse the first part at same time
    18. Delete Node in a linkedList
      Without giving the head of list
      Move the value around
    19. Odd Even LinkedList
      Separate with two list and combine them
    20. Reverse Linked List


    1. Insertion Sort List
    2. Sort List
      Merge sort and quick sort
    3. Largest Number String
      redefine comparator (s1s2 and s2s1)
    4. Maximum Gap
      Bucket Sort concept, max gap happens between buckets

    Binary Search and Sorted Array

    Binary Search Template

    start = 0; end = len - 1;
    while(start + 1 < len){
    mid = start + (end - start) / 2;
    if(num[mid] == target)
    return mid;
    if(target num[mid])
    start = mid;
    end = mid;

    num[start], num[end] ? target

    1. Median of Two Sorted Array
      Convert to find k-th element in array
      Compare the medians of two array and halve
      Find Minimum in Rotated Sorted Array (No Dup)
      Compare mid with end (consider the input array can be increasing order)
      Find Minimum in Rotated Sorted Array II (has Dup)
      o(n) — Blackbox test
    2. Search in rotated sorted Array, No dup
      Scenarios with mid in the first half or second half
    3. Search in rotated sorted Array, has dup
      o(n) — Blackbox test
    4. Search for a range
      Twice search for the startIndex and endIndex
    5. Search insert Position
      Insert Position: first position larger than target
    6. Search a 2D matrix (strictly increase)
      Binary Search for row and column or 2D to array
    7. Find peak element (no Dup)
      nums[mid] > nums[mid + 1], peak is in left half
      nums[mid] < nums[mid + 1], peak is in right half
      Find Peak element in 2D matrix
      Find the peak in mid row num[mid][j], compare with num[mid + 1][j]

    Search a 2D matrix II (row increase, column increase)
    a[i][j] < target,向右走,a[i][j] > target, 向上走 O(m + n)
    H-Index II:
    Input is sorted, compare with the num[mid] (citation count) and length - mid (number of paper has larger citation count)

    First Bad Version
    Test if mid is bad version and move pointer accordingly
    Find the Duplicate number
    Given n + 1 number and value is in range: [1, n]. Assume only one dup exists, find it.
    值的二分,统计元素在[start, mid], [mid + 1, end]的分布决定start和end的走向

    Binary Tree
    Binary Tree In-order Traversal
    Non-recursion solution
    Validate Binary Search Tree
    Binary Tree In-order traversal
    Recover Binary Search Tree
    Two elements are swap in BST, recover it
    Binary Tree In-order traversal
    Same Tree
    Given two trees, is the same or not
    Symmetric Tree
    Convert to isMirror(root.left, root.right)
    isMirror(A, B) <==> A.val == B.value && isMirror(A.left, B.right) && isMirror(A.right, B.left)
    Binary Tree Level Order Traversal I, II
    Queue and queueSize in each level
    Binary Tree ZigZag Level Order Traversal
    Queue and queueSize in each level
    Maximum Depth of a Binary Tree
    maxDep(root) = Max(maxDep(root.left), maxDep(root.right)) + 1
    Minimum Depth of a Binary Tree
    minDep(root) = Min(minDep(root.left), minDep(root.right)) + 1
    Construct Binary Tree From Preorder and In-order traversal
    Partition Inorder array with Preorder value (left to right)
    Construct Binary Tree From Postorder and In-order traversal
    Partition Inorder array with Postorder value (right to left)
    Convert Sorted Array to Binary Search Tree(Balance Height)
    Use mid index as root, left part as left tree, right part as right tree
    Convert Sorted List to Binary Search Tree(Balance Height)
    Use mid node as root, left part as left tree, right part as right tree
    Balanced Binary Tree
    isBalance(root) <==> isBalance(root.left) && isBalance(root.right) && height(left), height(right) 相差最多1
    Path Sum
    has PathSum = target
    Path Sum II
    return all PathSum = target, need DFS
    Flatten Binary tree to LinkedList
    Tree traverse with non-Recursive way
    Populating next right pointers in each node
    In-order traverse, non-recursive
    Binary Tree Maximum Path Sum
    Recursively return the max Value end with root, meanwhile use global value to record the max value from leaf to leaf
    Sum root to leaf number
    static variable to record the sum
    sumNumber(root, termSum) will recursively consider sumNumber(root.left,10 * (termSum + root.val)) and sumNumber(root.right, 10 * (termSum + root.val))
    Binary Tree Preorder Traversal
    Non-recursive way
    Binary Tree Postorder Traversal
    Recursive way should be fine
    Binary Search Tree Iterator
    In-order traversal, non-recursive
    Binary Tree Right Side View
    Level-order traversal, queueSize as level size
    Count Complete Tree Nodes
    Count number of nodes in complete tree.
    Turn into count the number of leaves. 值二分法查看mid是否是一个leaf
    Invert Binary Tree (左右交换)
    Invert(root) <=> leftInvert = Invert(root.left); rightInvert = Invert(root.right); root.left = rightInvert; root.right = leftInvert;

    Nth Smallest Element in BST
    Binary Tree in-order traverse, non-recursive

    Least Common Ancestor of Binary Tree
    如果root 下面有node 1, node 2, return LCA; 2. 如果root下面只有node 1 或node 2, return node1 or node2; 3. 如果root下面有没有, return null

    Binary Tree Paths
    打印所有Root to leaf paths. DFS, possible next steps are left tree and right tree

    Unique Binary Search Tree II
    Given an integer n, generate all structurally unique BST’s that store values 1…n
    Generate possible left tree and right tree and crossing combine those

    Serialize and Deserialize Binary Tree
    In-order traverse with null node represent by n, Each term split by ‘,’

    Count of Smaller Numbers After Self
    Use balance Binary Search Tree: TreeMap

    Count of range Sum
    PreSum: sum(i to j) = preSum[j] - preSum[i - 1], contract BST using PreSum
    iterate preSum array: remove preSum[i] and search for number of element in [preSum[i] + lower, preSum[i] + upper)

    Verify Preorder serialization of a Binary Tree
    9,3,4,#,#,1,#,#,2,#,6,#,# is a Preorder of Binary Tree or not
    keep removing X, #, # by #, to see if the array can be transform to # or not

    Data Stream as Disjoint Intervals
    Balance Binary Search Tree to search for lower and upper for the new given element, combine interval if needed

    从任意一点(root)出发, BFS走到最远far-end, 再从far-end出发,BFS到最远即为答案。

    Remove node in BST (hard!)
    Find the node, 2. Find 左子树最大值替换node. 3. 递归删除左子树最大值


    Letter Combination of a phone number:
    Use static string array for dictionary

    Generate Parenthesis
    Maintain leftCount and rightCount, the possible next step is if leftCount < n, add ‘(‘, if rightCount < leftCount, add ’)’

    Sudoku Solver
    For each position, from 1 to 9, decide whether it is valid next steps

    Combination Sum (可重复)
    Each candidate can be possible next step

    Combination Sum (不可重复)
    Need to detect duplicate

    Permutation (Inputs are distinct number)
    Elements which do not in current term are possible next step

    Permutation II (inputs may have same number)
    Need to record visited

    Put queue row by row, for each row, choose the valid place as next step.

    N-Queue II
    Put queue row by row, mark the board once queue is placed, unmark the board once backtracking

    Given two integers n and k, return all possible combinations of k numbers out of 1 … n
    Recursion stops with K, and next possible steps are index to n.

    Each node in recursion tree is valid answer

    Subsets II (input has duplicate)
    Sort input array and skip same element when finding next step

    Word search
    Find the first char in Matrix, possible next steps are up, down, left, right and match the next char in target

    Gray code
    n-level gray code is the n-bit set to 1 of n - 1 level gray code

    Restore IP address (Lottery Number)
    Next steps are 1 digit, 2 digit 3 digit less than 255.
    Discuss with ‘0’ (one next step), ‘1’ (three next steps), ‘2’ (two or three next steps), ‘3’ to ‘9’ (two next steps)

    Path Sum II (Binary Tree, all path Sum = target)
    Possible next step: left tree, right tree
    Palindrome Partitioning
    Next possible steps: All the valid palindrome prefix, need a function to find those

    Combination Sum III
    Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and no dup
    index (dup prevent) to 9 are possible next steps

    Flatten Nested List Iterator

    Word Ladder I/II


    Valid Parentheses
    Maintain left parentheses stack, if left parentheses, push to stack, if right parentheses, pop. If nothing to pop, invalid. In the end, if stack not empty, invalid

    Largest Rectangle in Histogram
    Maintain a increasing stack, iterate list, a[i] will pop all the stack elements greater than a[i], once a stack element poped, the left boundary of rectangle is index of stack.peek(), right boundary is i.
    Maximal Rectangle
    For each row, solve the Largest Rectangle in Histogram problem

    Evaluate Reverse Polish Notation
    Maintain a number stack, iterate the input list, if number, push to stack, if operator, pop two element from stack as operands and push result back to stack

    Basic Calculator
    Firstly convert to reverse polish notation, and then Evaluate Reverse Polish Notation
    To RPN: Maintain an operator stack, iterate list, if a[i]是左括号, push; if a[i]是右括号, pop until 左括号, if a[i]是加减, pop until 左括号; if a[i]是乘除, pop until 加,减或左括号

    Implement Stack with Queues
    Two queues, one empty, one full
    Push: enqueue to full queue
    Pop: dequeue from full queue and enqueue to empty queue until last element, return last element
    Peek: dequeue from full queue and enqueue to empty queue until last element, last element is the result and then enqueue it as well

    Implement Queue with Stack
    In stack and out stack, in stack has the queue’s order, out stack has the queue’s reverse order
    offer: if outstack is empty, just push to instack, if not, pop outstack and push to in stack and then push input
    poll: if outstack is not empty, just pop outstack, if not, pop in stack and push to outstack, and pop
    peek: same as poll

    Min Stack
    Maintain two stacks, one is the normal stack for the basic stack functionality and one is min Stack maintains element in decreased order, minStack.peek() is the min value

    Remove Duplicate Letters
    Given “cbacdcbc”, return “acbd”
    Maintain the stack to record the actual result. If current is smaller than top of stack and the top of stack count great than 0, which means the top of stack should use the one who will appear later, pop the top of stack. Every current should then push to stack


    Sliding windows maximum
    Maintain a deque with index, so that deque.peek is the index of sliding windows maximum (return num[deque.peek())
    For num[i], remove out of windows range value from head of queue; then remove all the value less than num[i] from tail; then enqueue[i]
    小技巧,当既需要index有需要value的时候,将index enqueue即可

    Substring with concatenation of all words:
    Map for the word count. Construct global map for the input words dictionary, then start with each character in string, cut the following string with same number, same length words and use a second Map for word count to compare with input dictionary.

    Valid Sudoku
    Set for dup detection in row, column and 9-cells.

    Group Anagrams
    Map for grouping, key is the sorted char array for a given word (same for anagrams) and value is the index for given word

    Max Points on a line
    Map for grouping

    Happy Number
    Use Map to detach if same state happened again or not

    Isomorphic String
    Map for maintaining/deciding valid 1 to many mapping. To validate 1 to 1 mapping, need to valid s—>t and t—>s, twice or validate the value Set also has no dup

    Word pattern
    Same as Isomorphic String

    Contains Duplicate
    Determine an int array has dup or not
    Set for dup detection

    Contains Duplicate II
    Determine an int array, nums[i] = nums[j] and the difference between i and j is at most k.
    Set with max Size = k for dup detection
    Contains Duplicate III
    Determine an int array, nums[i] diff nums[j] at most t and the difference between i and j is at most k.
    TreeSet with max size = k, for a given nums[i], check the floor and celling with nums[i] +/- t

    Valid Anagram
    Use int[26] as char count, compare the char count

    Bulls and Cons
    Bulls count is a[i] == b[i]. Cons count is related to digit count

    Top K Frequent Elements
    Map for freq count and heap for find K largest value

    Intersection of two arrays (No dup in result)
    Collect the dup element, set for dup detection. Create Set from first array and find the dup in second array

    Intersection of two arrays II (has Dup in result)
    Map for dup detection and freq count.

    Repeated DNA Sequences
    HashMap to count the 10-letter sequence frequency

    Longest Consecutive Sequence
    Insert all the array elements to Set, for each element, check if element - 1, element - 2 … and element + 1, element + 2 … in the Set or not, maxLen can be obtained among all elements. Note that if element - 1, element - 2 … element + 1, element + 2 … exist in Set, need to remove them to ensure O(n) time complexity.

    Longest Palindromic Substring
    O(n^2)算法: 找出以每个char为中点的回文substring,需要分奇偶数讨论

    Zigzag Conversation
    Simulate the whole process

    Longest Common Prefix among String array
    Use n-pointer compare

    Implement strStr
    O(n^2) solution, careful for the boundary check, KMP is not required

    Count and Say
    Just Count and Say n times

    Length of last word
    Once a word is finished, update the result value

    Valid Palindrome
    Two pointers from start and end and compare
    Reverse Words in a string
    Double reverse, reverse the entire string and then reverse each word (separate by token)

    Compare version number
    Split version string with token (string array 1 and string array 2) and compare array 1 and array 2 from 0 to n

    Shortest Palindrome
    O(n2), move the axis from middle to begin, padding the prefix for each axis and find the minimum padding. 分奇偶讨论

    Reverse String
    Be careful that string is immutable, need new space for reverse

    Reverse Vowels of a String
    Head and tail pointers, find the vowel in head and vowel in tail, swap


    Two Pointer 类

    1. 相对型
      Two Sum
      Sort array and two pointers from head and tail moving towards

    Container with most water
    Head and tail pointer, 那个小就往中间移动, 最大值一定在这些Head/Tail的组合中

    Sort array and fix one num, two pointers from head and tail moving towards (inner loop using Two Sum), records all possible solution

    3Sum closet
    Sort array and fix one num, two pointers from head and tail moving towards (inner loop using Two Sum), record the closest solution

    For(for(twoSum)) or Hashtable to store 2Sum combo

    Trapping Rain Water/Trapping Rain Water II
    The water level is defined by the lower one of head/tail. head,tail哪个小往中间移动(两种情况),可装的水由curLevel - nums[i] 决定

    Merge Interval
    Sort Interval with starting point, determine the overlap and combine interval with overlap

    Insert Intervals
    Find proper position for the inserted Interval (Previous Interval end >= inserted interval start, Binary Search might be used), then merge the overlap interval (find the end of interval)

    Sort colors (三色问题)
    on-line algorithm, scan the next input (index), [0, i] is 0, (i, j] is 1 and (j, index] is 2, keep updating i, j given input as 0, 1 or 2

    1. 追击型
      Longest Substring Without Repeating Characters
      Sliding windows with fast/slow pointer, use Set to dup detect

    Minimum Window Substring
    Sliding windows with fast/slow pointer, use int array to count the number of each char

    Minimun Size Subarray Sum (positive input)
    Sliding windows with fast/slow pointer, fluctuate around target

    Scanning Line Problem
    The Skyline Problem
    Create Event for (location, isStart) sort the event based on location. Process each event as scanning line move from left to right

    Remove Duplicates from sorted Array
    Reuse the same array, newArrayIndex to record the end of new Array

    Remove Duplicates from sorted Array II (dup are allowed twice)
    Reuse the same array, newArrayIndex to record the end of new Array, state has isTwice

    Remove Element
    Reuse the same array, newArrayIndex to record the end of new Array

    First Missing positive
    Space reuse: Suppose you have an int array appear, appear[i] = 1 means i is appeared. Need to reuse nums space for appear
    Encode: s[i] = appear[i] * mode + nums[i]; appear[i] = s[i] / mode; nums[i] = s[i] % mode
    (mode = array length)

    Mege sorted Array
    Reuse the space in array1 and merge backward.

    Move Zeroes
    Reuse the same array, newArrayIndex to record the end of new Array

    Next Permutation
    From the end and iterate forward, find first a[i - 1] < a[i], leave a[0] to a[i - 1] untouched
    From the end, find the first number larger than a[i - 1], swap with a[i - 1]
    sort a[i] to end

    Pascal’s Triangle (打印n阶杨辉三角)

    Pascal’s Triangle II(打印nth row)
    Rolling array to save space since nth row just related to n-1th row

    Majority Element
    Maintain majority element and majority element count, if cur = majority element, count ++, else count—, update majority element once count = 0

    Majority Element II
    Same as Majority Element, need to keep majority element 1, 2, count1, 2.

    Rotate Array
    三步翻转,翻转整体,翻转前半,翻转后半 或 翻转前半,翻转后半,翻转整体

    Nth largest element in an Array
    Quick select: partition an array and choose first half or second half based on rank of pivot

    Summary Ranges
    Given [0,1,2,4,5,7], return [“0->2”,“4->5”,“7”]
    Use cur and next two pointers to check if elements are consecutive. Update the start/end for each term

    Game of life
    Simulate the entire process and reuse space by redefine the state

    Wiggle Sort II
    Partition array and interleaving first part and second part

    Divide and conquer

    Different ways to add Parentheses
    Divide and conquer: Divide array with each operator, recursively resolve left part and right part, then combine left and right

    Product of Array Except self
    Divide the array in the middle, recursively return the all element product of left and right, combine left and right by: for each left element, multiply the product of right part and for each right element, multiply the product of left part. For single element case, set the result to 1(except self)

    Union - Find
    Surrounded Regions

    Number of island


    Clone graph

    Reconstruct Itinerary
    DFS: next possible steps are those ticket who’s orig is current place, access next possible steps with lexical order

    Course Schedule I, II
    Topology sort:
    BFS, maintain a queue with in-grade 0 node, when processing the node, update the in-grade of the next node, enqueue those node who’s in-grade become 0. If visited all nodes, no cycle.

    Tree structure, Node has: word, isWord and hashMap<nextChar, nextTrieNode>.
    Operation: Insert, searchWord, searchPrefix, delete

    Implement Trie
    Insert: iterative
    searchWord/searchPrefix: iterative
    delete: recursive

    Add and Search Word - Data structure design
    Construct Trie to addWord, searchWord, once wildcard ‘.’ happens, searchWord with DFS(back -tracing)

    Word Search Two
    Construct Trie for dictionary, in the board, find the possible first letter and start DFS. Once a word is found, need to delete it from Trie (dup prevent)

    HashTable + DoublyLinkedList

    Peeking iterator
    Consume one more element for peeking

    Design Twitter
    Push or Poll


    Jump Game
    Jump GameII
    Best time buy and sell stock II (No time limitation)
    Gas Station
    Patching Array

  • wow


  • 加油。指不定抽獎就中了呢。

  • 这人品攒的 杠杠的 中奖几率Up↑


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