LeetCode-112. Path Sum

問題：https://leetcode.com/problems/path-sum/?tab=Description
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. 給定一個二叉樹和一個值sum，判斷是否存在一個從根節點到葉子節點的路徑，使得路徑上每個節點值之和等於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.
分析：遞歸，判斷各個路徑是否有滿足條件、
參考C++代碼：

/**
 * Definition for a binary tree node.
 * 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(NULL==root) return false;
        if(NULL==root->left && NULL==root->right && sum==root->val) return true;
        return hasPathSum(root->left,sum-root->val) || hasPathSum(root->right,sum-root->val);
    }
};