LeetCode 222. 完全二叉树节点个数（方法一）

/**
 * 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 countNodes(TreeNode* root) {
      if(!root){
          return 0;
      }
      return 1+countNodes(root->left)+countNodes(root->right);
    }
};