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Question:

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

5             / \            4   8           /   / \          11  13  4         /  \      \        7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

题目要求在树中找到一条总和等于Sum的路径，等价于在树的左右两个子树中找到和等于sum-root的路径，这样依次递归。

代码如下：

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool hasPathSum(TreeNode *root, int sum) {	if (root==NULL)		return false;	if(root->left ==NULL && root->right==NULL)		return sum == root->val;	if (root->left && hasPathSum(root->left, sum-root->val))		return true;	if (root->right && hasPathSum(root->right, sum-root->val))		return true;	return false;    }};

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