2023.7.3

        用层序遍历遍历一遍二叉树，然后遍历的每个节点都进行一次计数，直接上代码：

class Solution {
public:
    int countNodes(TreeNode* root) {
        queue<TreeNode*> que;
        int ans = 0;
        if(root == nullptr) return ans;
        que.push(root);
        while(!que.empty())
        {
            int size = que.size();
            while(size--)
            {
                TreeNode* node = que.front();
                que.pop();
                ans++;
                if(node->left) que.push(node->left);
                if(node->right) que.push(node->right);
            }
        }
        return ans;
    }
};