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 and sum = 22,

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.

思考：深度优先遍历。

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void DFS(TreeNode *root,int sum,int ans,bool &flag)    {        if(root)        {            ans+=root->val;            if(ans==sum&&!root->left&&!root->right)            {                flag=true;            }            DFS(root->left,sum,ans,flag);            DFS(root->right,sum,ans,flag);        }    }    bool hasPathSum(TreeNode *root, int sum) {        int ans=0;        bool flag=false;        DFS(root,sum,ans,flag);        return flag;    }};