[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.

这是个一目了然的深度优先搜索题，有几点需要注意：

1. 树里可能出现负数

2.当 sum - node->val == 0 时不一定就是叶子节点

    bool hasPathSum(TreeNode *root, int sum) {
        if (!root) return false;
        int val = sum - root->val;
        if (val == 0 && !root->left && !root->right) {
                return true;
        }
        return hasPathSum(root->left, val) || hasPathSum(root->right, val);

    }