二叉树：节点的个数

二叉树：节点的个数

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