LeetCode[Tree]: 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,

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

这个题目非常适合用递归来做，C++代码实现非常简单：

    bool hasPathSum(TreeNode *root, int sum) {
        if (!root) return false;

        if (root->left  && hasPathSum(root->left,  sum - root->val)) return true;
        if (root->right && hasPathSum(root->right, sum - root->val)) return true;
        if (root->left == nullptr && root->right == nullptr && root->val == sum) return true;

        return false;
    }

时间性能如下图所示：