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：
LeetCode: Maximum Depth of Binary Tree,布布扣,bubuko.com
LeetCode: Maximum Depth of Binary Tree
原文：http://www.cnblogs.com/yeek/p/3585434.html