text: 题目描述
  
  
  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. 
  
  /**
   * Definition for binary tree
   * struct TreeNode {
   *     int val;
   *     TreeNode *left;
   *     TreeNode *right;
   *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
   * };
   */
  //时间复杂度为O(n),空间复杂度为(logn)
  class Solution {
  public:
      int maxDepth(TreeNode *root) {
          if(root == nullptr) return 0;
          return max(maxDepth(root->left) , maxDepth(root->right)) + 1;
      }
  };
  