[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;

    }

};