题目:

 

 

 

解答：

声明一个变量记录已经经过的节点的值之和，每经过一个节点就加上这个节点的值，在叶子节点判断变量值是否为目标值。

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool hasPathSum(TreeNode* root, int sum) 
13     {
14         return helper(root, 0, sum);        
15     }
16 
17     bool helper(TreeNode * root, int cur, int sum)
18     {
19         if (NULL == root)
20         {
21             return false;
22         }
23 
24         cur = cur + root->val;
25         
26         if (root->left == NULL && root->right == NULL)
27         {
28             return cur == sum;
29         }
30         else
31         {
32             return helper(root->left, cur, sum) || helper(root->right, cur, sum);
33         }
34     }
35 };