text: /**
   * Definition for a binary tree node.
   * struct TreeNode {
   *     int val;
   *     TreeNode *left;
   *     TreeNode *right;
   *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
   * };
   */
  class Solution {
  public:
      int maxDepth(TreeNode* root) {
          if(root == nullptr) return 0;
          return max(maxDepth(root->left),maxDepth(root->right))+1;
      }
  };