112. Path Sum

问题描述：

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：

递归，递减sum，判断root->val是否为sum，有则返回true.

class Solution {
public:
    bool hasPathSum(TreeNode* root, int sum) {
        if(root==NULL) return NULL;
        else if(root->left==NULL&&root->right==NULL&&root->val==sum)
            return true;
        else {return hasPathSum(root->left,sum-root->val)||hasPathSum(root->right,sum-root->val);}
    }
};