Problem Link: http://oj.leetcode.com/problems/path-sum-ii/ The basic idea here is same to that of Path Sum. However, since the problem is asking for all possible root-to-leaf paths, so we should use BFS but not DFS. The python code is as follows. # D…

Problem Link: http://oj.leetcode.com/problems/path-sum/ One solution is to BFS the tree from the root, and for each leaf we check if the path sum equals to the given sum value. The code is as follows. # Definition for a binary tree node # class TreeN…

题目:Given an array of integers, find two numbers such that they add up to a specific target number. The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note th…

Problem Link: http://oj.leetcode.com/problems/interleaving-string/ Given s1, s2, s3, find whether s3 is formed by the interleaving of s1 and s2. For example, Given: s1 = "aabcc", s2 = "dbbca", When s3 = "aadbbcbcac", return t…

Problem link: http://oj.leetcode.com/problems/reverse-words-in-a-string/ Given an input string, reverse the string word by word. For example, Given s = "the sky is blue", return "blue is sky the". LeetCode OJ supports Python now! The s…

Problem Link: http://oj.leetcode.com/problems/binary-tree-maximum-path-sum/ For any path P in a binary tree, there must exists a node N in P such that N is the ancestor node of all other nodes in P. We call such N as the root of P, or P roots at N. T…

# Definition for a binary tree node # class TreeNode: # def __init__(self, x): # self.val = x # self.left = None # self.right = None class Solution: # @param root, a tree node # @return an integer def sumNumbers(self, root): """ BFS the tre…

Problem Link: http://oj.leetcode.com/problems/triangle/ Let R[][] be a 2D array where R[i][j] (j <= i) is the minimum sum of the path from triangle[0][0] to tirangle[i][j]. We initialize R[0][0] = triangle[0][0], and update R[][] from i = 1 to n-1: R…

Problem link: http://oj.leetcode.com/problems/gas-station/ We can solve this problem by following algorithm. CompleteCircuit(gas, cost): let n be the number of stations // Check if there exists i such that A[i,n] >= 0 if (sum of gas) > (sum of cost)…

Problem link: http://oj.leetcode.com/problems/candy/ Suppose we are given an array R[1..N] that are ratings of N children. Let C[1..N] be a new array where C[i] is the number of candies for i-th child. From the description, C[i] should meet following…

Problem Link: https://oj.leetcode.com/problems/validate-binary-search-tree/ We inorder-traverse the tree, and for each node we check if current_node.val > prev_node.val. The code is as follows. # Definition for a binary tree node # class TreeNode: #…

Problem Link: https://oj.leetcode.com/problems/recover-binary-search-tree/ We know that the inorder traversal of a binary search tree should be a sorted array. Therefore, we can compare each node with its previous node in the inorder to find the two…

Problem Link: https://oj.leetcode.com/problems/same-tree/ The following recursive version is accepted but the iterative one is not accepted... # Definition for a binary tree node # class TreeNode: # def __init__(self, x): # self.val = x # self.left =…

Problem Link: https://oj.leetcode.com/problems/symmetric-tree/ To solve the problem, we can traverse the tree level by level. For each level, we construct an array of values of the length 2^depth, and check if this array is symmetric. The tree is sym…

Problem Link: https://oj.leetcode.com/problems/binary-tree-level-order-traversal/ Traverse the tree level by level using BFS method. # Definition for a binary tree node # class TreeNode: # def __init__(self, x): # self.val = x # self.left = None # se…

Problem Link: https://oj.leetcode.com/problems/binary-tree-zigzag-level-order-traversal/ Just BFS from the root and for each level insert a list of values into the result. # Definition for a binary tree node # class TreeNode: # def __init__(self, x):…

Problem Link: https://oj.leetcode.com/problems/maximum-depth-of-binary-tree/ Simply BFS from root and count the number of levels. The code is as follows. # Definition for a binary tree node # class TreeNode: # def __init__(self, x): # self.val = x #…

Problem Link: https://oj.leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/ The basic idea is same to that for Construct Binary Tree from Inorder and Postorder Traversal. We solve it using a recursive function. First, we…

Problem Link: https://oj.leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/ This problem can be easily solved using recursive method. By given the inorder and postorder lists of the tree, i.e. inorder[1..n] and postorde…

Problem Link: https://oj.leetcode.com/problems/binary-tree-level-order-traversal-ii/ Use BFS from the tree root to traverse the tree level by level. The python code is as follows. # Definition for a binary tree node # class TreeNode: # def __init__(s…

Problem Link: http://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/ Same idea to Convert Sorted Array to Binary Search Tree, but we use a recursive function to construct the binary search tree. # Definition for a binary tree nod…

Problem Link: http://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/ We design a auxilar function that convert a linked list to a node with following properties: The node is the mid-node of the linked list. The node's left child i…

Problem Link: http://oj.leetcode.com/problems/balanced-binary-tree/ We use a recursive auxilar function to determine whether a sub-tree is balanced, if the tree is balanced, it also return the depth of the sub-tree. A tree T is balanced if the follow…

Problem Link: http://oj.leetcode.com/problems/minimum-depth-of-binary-tree/ To find the minimum depth, we BFS from the root and record the depth. For each level we add 1 to the depth and return the depth value when we reach a leaf. The python code is…

Problem Link: http://oj.leetcode.com/problems/flatten-binary-tree-to-linked-list/ The problem is asking for flatterning a binary tree to linked list by the pre-order, therefore we could flatten tree from the root. For each node, we link it with its n…

Problem Link: http://oj.leetcode.com/problems/distinct-subsequences/ A classic problem using Dynamic Programming technique. Let m and n be the length of the strings T and S. Let R[i][j] be the count of distinct subsequence of T[0..i] in S[0..j]. Obvi…

Problem Link: http://oj.leetcode.com/problems/populating-next-right-pointers-in-each-node-ii/ OK... Exactly same to Populating Next Right Pointers in Each Node.…

Problem Link: http://oj.leetcode.com/problems/populating-next-right-pointers-in-each-node/ Just traverse the tree from the root, level by level. Esay enough. # Definition for a binary tree node # class TreeNode: # def __init__(self, x): # self.val =…

Problem Link: http://oj.leetcode.com/problems/pascals-triangle-ii/ Let T[i][j] be the j-th element of the i-th row in the triangle, for 0 <= j <= i, i = 0, 1, ... And we have the recursive function for T[i][j] T[i][j] = 1, if j == 0 or j == i T[i][j…

Prolbem Link: http://oj.leetcode.com/problems/pascals-triangle/ Just a nest-for-loop... class Solution: # @return a list of lists of integers def generate(self, numRows): # Initialize the triangle res = [] for i in xrange(numRows): res.append([1] * (…