path-sum

题目：

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.
For example:
Given the below binary tree andsum = 22,
5
/ \
4 8
/ / \
11 13 4
/ \ \
7 2 1
return true, as there exist a root-to-leaf path5->4->11->2which sum is 22.

程序：

class Solution {public:    bool hasPathSum(TreeNode *root, int sum) {        if(root==NULL)            return false;        bool flag=false;        solve(flag,root,sum);        return flag;    }    void solve(bool &flag, TreeNode *root,int sum)    {        if(root&&root->left==NULL&&root->right==NULL&&sum==root->val)        {            flag=true;            return;        }        if(root==NULL)            return;        solve(flag,root->left,sum-root->val);        solve(flag,root->right,sum-root->val);    }};