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LeetCode Notebook
  • Introduction
  • Useful Java knowledge
    • Arrays vs Math vs Collections
    • Integer vs int
    • String vs string vs char vs Character
    • StringBuilder vs StringBuffer
    • ArrayList vs Array
    • Stack & Queue
    • HashMap
    • LinkedList
  • Useful Python knowledges
    • Dictionary
    • recursive vs iterative
    • queue 和 stack如何用python实现
    • List, Dictionary, Set 常用的函数
    • Common Mistakes about Python
    • Bit operation
    • sort by second value in Python
    • Initialize list in Python
  • 完全按照九章算法刷的,70道左右
    • I. Binary Search
      • 34.Find First and Last Position of Element in Sorted Array
      • 35. Search Insert Position
      • 74. Search a 2D Matrix
      • 240. Search a 2D Matrix II
      • 278. First Bad Version
      • 162. Find Peak Element
      • 33. Search in Rotated Sorted Array
      • 81. Search in Rotated Sorted Array II
      • 153. Find Minimum in Rotated Sorted Array
      • 154. Find Minimum in Rotated Sorted Array II
    • II. Sorted Array
      • 88. Merge Sorted Array
      • 23. Merge k Sorted Lists
      • 4. Median of Two Sorted Arrays
      • # Recover rotated sorted array
      • 796. Rotate String
      • 26. Remove Duplicates from Sorted Array
      • 80. Remove Duplicates from Sorted Array II
      • 557. Reverse Words in a String III
    • III. Binary Tree
      • 144. Binary Tree Preorder Traversal
      • 94. Binary Tree Inorder Traversal
      • 145. Binary Tree Postorder Traversal
      • 912. Sort an Array
      • 104. Maximum Depth of Binary Tree
      • 110. Balanced Binary Tree
      • 124. Binary Tree Maximum Path Sum
      • 235. Lowest Common Ancestor of a Binary Search Tree
      • 102. Binary Tree Level Order Traversal
      • 107. Binary Tree Level Order Traversal II
      • 103. Binary Tree Zigzag Level Order Traversal
      • 98. Validate Binary Search Tree
      • 701. Insert into a Binary Search Tree
      • 173. Binary Search Tree Iterator
    • IV. Permutations and Subsets
      • 78. Subsets
      • 90. Subsets II
      • 31. Next Permutation
      • 60. Permutation Sequence
      • 51. N-Queens
      • 17. Letter Combinations of a Phone Number
      • 39. Combination Sum
      • 131. Palindrome Partitioning
      • 126. Word Ladder II
      • 127. Word Ladder
    • VI. Linked List
      • 203. Remove Linked List Elements
      • 206. Reverse Linked List
      • 92. Reverse Linked List II
      • 143. Reorder List
      • 148. Sort List
      • 82. Remove Duplicates from Sorted List II
      • 86. Partition List
      • 141. Linked List Cycle
      • 142. Linked List Cycle II
      • 23. Merge k Sorted Lists
      • 138. Copy List with Random Pointer
    • IX. Dynamic Programming
      • 70. Climbing Stairs
      • 120. Triangle
      • 62. Unique Paths
      • 63. Unique Paths II
      • 64. Minimum Path Sum
      • 55. Jump Game
      • 300. Longest Increasing Subsequence
      • 139. Word Break
      • 132. Palindrome Partitioning II
      • 72. Edit Distance
      • 1143. Longest Common Subsequence
      • 97. Interleaving String
      • 115. Distinct Subsequences
  • 开始自己刷,冲刺100道,主要是binarySearch&Tree
    • 981. Time Based Key-Value Store
    • 111. Minimum Depth of Binary Tree
    • 112. Path Sum
    • 113. Path Sum II
    • 100. Same Tree
    • 101. Symmetric Tree
    • 96. Unique Binary Search Trees
    • 108. Convert Sorted Array to Binary Search Tree
    • 226. Invert Binary Tree
    • 95. Unique Binary Search Trees II
    • 50. Pow(x, n)
    • 69. Sqrt(x)
    • 167. Two Sum II - Input array is sorted
    • 29. Divide Two Integers*
    • 349. Intersection of Two Arrays
    • 287. Find the Duplicate Number
    • 222. Count Complete Tree Nodes
    • 350. Intersection of Two Arrays II
    • 257. Binary Tree Paths
    • 404. Sum of Left Leaves
    • 437. Path Sum III
    • 7. Reverse Integer
    • 21. Merge Two Sorted Lists
    • 279. Perfect Squares
    • 199. Binary Tree Right Side View
    • 114. Flatten Binary Tree to Linked List
    • 129. Sum Root to Leaf Numbers
    • 198. House Robber
    • 213. House Robber II
    • 337. House Robber III
    • 236. Lowest Common Ancestor of a Binary Tree
  • 终于刷到100道了,我是分水岭,开始按照网上推荐重点250道题
    • 补充1-20道
      • 3. Longest Substring Without Repeating Characters
      • 5. Longest Palindromic Substring
      • 9. Palindrome Number
      • 11. Container With Most Water
      • 12. Integer to Roman
      • 13. Roman to Integer
      • 15. 3Sum
      • 18. 4Sum
      • 20. Valid Parentheses
      • 22. Generate Parentheses
      • 27. Remove Element
      • 28. Implement strStr()
      • 31. Next Permutation
      • 36. Valid Sudoku
      • 38. Count and Say
      • 40. Combination Sum II
      • 41. First Missing Positive
      • 43. Multiply Strings
      • 48. Rotate Image
      • 49. Group Anagrams
  • 补充21-40道
    • 53. Maximum Subarray
    • 66. Plus One
    • 67. Add Binary
    • 71. Simplify Path
    • 75. Sort Colors
    • 77. Combinations
    • 79. Word Search
    • 91. Decode Ways
    • 116. Populating Next Right Pointers in Each Node
    • 117. Populating Next Right Pointers in Each Node II
    • 121. Best Time to Buy and Sell Stock
    • 122. Best Time to Buy and Sell Stock II
    • 125. Valid Palindrome
    • 133. Clone Graph
    • 134. Gas Station
    • 146. LRU Cache
    • 150. Evaluate Reverse Polish Notation
    • 152. Maximum Product Subarray
    • 155. Min Stack
    • 157. Read N Characters Given Read4
  • 补充41-60道
    • 160. One Edit Distance
    • 163. Missing Ranges
    • 168. Excel Sheet Column Title
    • 169. Majority Element(位运算)
    • 170. Two Sum III - Data structure design
    • 171. Excel Sheet Column Number
    • 175. Combine Two Tables(SQL)
    • 176. Second Highest Salary(SQL)
    • 177. Nth Highest Salary(SQL)
    • 178. Rank Scores(SQL)
    • 180. Consecutive Numbers(SQL)
    • 181. Employees Earning More Than Their Managers(SQL)
    • 182. Duplicate Emails(SQL)
    • 183. Customers Who Never Order(SQL)
    • 184. Department Highest Salary(SQL)
    • 185. Department Top Three Salaries(SQL)
    • 57. Insert Interval
    • 355. Design Twitter
    • 328. Odd Even Linked List(经典)
  • 补充61-80道
    • 378. Kth Smallest Element in a Sorted Matrix
    • 309. Best Time to Buy and Sell Stock with Cooldown
    • 186. Reverse Words in a String II
    • 190. Reverse Bits(bit)
    • 191. Number of 1 Bits(bit)
    • 196. Delete Duplicate Emails(SQL)
    • 197. Rising Temperature(SQL)
    • 200. Number of Islands
    • 201. Bitwise AND of Numbers Range
    • 202. Happy Number
    • 204. Count Primes
    • 205. Isomorphic Strings
    • 208. Implement Trie (Prefix Tree)
    • 211. Design Add and Search Words Data Structure
    • 215. Kth Largest Element in an Array
    • 216. Combination Sum III
    • 217. Contains Duplicate
    • 219. Contains Duplicate II
    • 220. Contains Duplicate III
    • 225. Implement Stack using Queues
  • 补充81-100道
    • 2. Add Two Numbers
    • 19. Remove Nth Node From End of List
    • 227. Basic Calculator II
    • 228. Summary Ranges
    • 231. Power of Two
    • 232. Implement Queue using Stacks
    • 234. Palindrome Linked List
    • 237. Delete Node in a Linked List
    • 242. Valid Anagram
    • 258. Add Digits
    • 263. Ugly Number
    • 268. Missing Number
    • 283. Move Zeroes
    • 290. Word Pattern
    • 303. Range Sum Query - Immutable
    • 26. Power of Three
    • 342. Power of Four
    • 344. Reverse String
    • 655. Print Binary Tree
    • 224. Basic Calculator
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  • Solution 1: Recursive
  • Solution 2: Divide & Conquer
  • Solution 3. Iterative(While-loop)

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  1. 完全按照九章算法刷的,70道左右
  2. III. Binary Tree

145. Binary Tree Postorder Traversal

# Hard, DFS

Solution 1: Recursive

Same logic as inorder and preorder binary traversal.

// Some code
class Solution {
    ArrayList<Integer> res = new ArrayList<>();
    public List<Integer> postorderTraversal(TreeNode root) {
        if(root == null)
            return res;
        
        postorderTraversal(root.left);
        postorderTraversal(root.right);
        res.add(root.val);
        return res;
    }
}
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def traversal(self, root, result):
        if root == None:
            return
        
        self.traversal(root.left, result)
        self.traversal(root.right, result)
        result.append(root.val)
        
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        # condier edge case
        result = []
        if root == None:
            return result
        
        # regular case
        self.traversal(root, result)
        return result

Solution 2: Divide & Conquer

Same logica as preorder and inorder binary traversal.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        # consider edge case
        result = []
        if root == None:
            return result
        
        # regular case
        # divide
        left = self.postorderTraversal(root.left)
        right = self.postorderTraversal(root.right)
        
        # conquer
        result.extend(left)
        result.extend(right)
        result.append(root.val)
        
        return result

Solution 3. Iterative(While-loop)

Use while-loop to find the most left node, adding them into the stack , remark those nodes as False at the same time( False means this node' right subtree hasn't been visited). Pop the most left node, if the node has been marked as True(the right subtree has been visited), then this node is root, add to res and put current ptr to None(in order to get parent node). Otherwise mark this node as visited, then push all the right subtree into the stack.

This is most efficient method.

// Some code
class Solution {
  public List<Integer> postorderTraversal(TreeNode root) {
    if (root == null)
      return new ArrayList<>();

    List<Integer> ans = new ArrayList<>();
    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);

    while (!stack.isEmpty()) {
      root = stack.pop();
      ans.add(root.val);
      if (root.left != null)
        stack.push(root.left);
      if (root.right != null)
        stack.push(root.right);
    }

    Collections.reverse(ans);
    return ans;
  }
}
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        stack = []
        
        while root != None or len(stack) != 0:
            while root != None:
                stack.append([root, False])
                root = root.left
            tmp = stack.pop()
            root = tmp[0]
            if tmp[1] == True:
                res.append(tmp[0].val)
                root = None
            else:
                tmp[1] = True
                stack.append(tmp)
                root = root.right
                
        return res
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