[LeetCode]Path Sum

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;
    }
};