题目链接/文章讲解/视频讲解：https://programmercarl.com/0222.%E5%AE%8C%E5%85%A8%E4%BA%8C%E5%8F%89%E6%A0%91%E7%9A%84%E8%8A%82%E7%82%B9%E4%B8%AA%E6%95%B0.html

1.这个题我寻思和前几天的遍历二叉树也没啥区别呀，但是看了视频我才发现题目强调的完全二叉树所以还有利用完全二叉树的性质的做法。

CPP

/**
 * 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 getNum(TreeNode *node){
        if(node == NULL) return 0;

        TreeNode * left = node->left;
        TreeNode * right = node->right;
        int leftDepth = 0,rightDepth = 0;
        while(left){
            left = left->left;
            leftDepth++;
        }
        while(right){
            right = right->right;
            rightDepth++;
        }
        if(leftDepth == rightDepth){
            return (2 << leftDepth) -1;
        }
        int leftSum = getNum(node->left);
        int rigthSum = getNum(node->right);
        return leftSum + rigthSum + 1;
    }
    int countNodes(TreeNode* root) {
        return getNum(root);
    }
};