958. Check Completeness of a Binary Tree

https://leetcode.com/problems/check-completeness-of-a-binary-tree/description/

检查二叉树是否complete。完全二叉树只有最后一层不完整，且最后一层只有右侧可能有连着的空缺。检查层信息，用BFS。完整二叉树seriliaze后中间不会出现null，利用这条性质在BFS时无论l和r是否为null都往q里插，当弹出遇到null后如果是complete的，BFS应该结束；否则不是complete的。

/*
 * @lc app=leetcode id=958 lang=cpp
 *
 * [958] Check Completeness of a Binary Tree
 */

// @lc code=start
/**
 * 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:
    bool isCompleteTree(TreeNode* root) {
        if (root == nullptr) return false;
        queue<TreeNode*> q;
        q.push(root);
        bool end = false;
        while (!q.empty()) {
            const auto t = q.front();
            q.pop();
            if (t == nullptr) end = true;
            else {
                if (end) return false;
                q.push(t->left);
                q.push(t->right);
            }
        }
        return true;
    }
};
// @lc code=end