LeetCode OJ

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 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.

解题思路：

1，若树为空返回false

2，若树只有一个根节点，返回 root->val == sum ?

3，若树没有左子树，在右子树中搜索

4，若树没有右子树，在左子树中搜索

5，否则，搜索左子树和右子树

代码：

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