LeetCode: Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

递归版本：

class Solution {
public:
    int maxDepth(TreeNode *root) {
        if(!root) return 0;  // !root
        else return max(maxDepth(root->left),maxDepth(root->right)) + 1;
    }
};

非递归版本：