958. 二叉树的完全性检验

/**
 * 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:
    bool isCompleteTree(TreeNode* root) {
        queue<TreeNode*>q;
        q.push(root);
        bool flag = false;
        while (!q.empty()) {
            TreeNode *node = q.front();
            q.pop();
            // 某个pop出的节点为空
            if (node == nullptr) {
                flag = true;
                continue;
            }
            // 某个pop出的节点为空，并且队列中仍然有非空的元素
            if (flag) return false;
            q.push(node->left);
            q.push(node->right);
        }
        return true;
    }
};