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.

dfs进行递归。

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