958. Check Completeness of a Binary Tree

O(n) time O(n) space

/**
 * 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) {
        queue<TreeNode *> q;
        q.push(root);
        while (!q.empty()) {
            auto n = q.front(); q.pop();
            if (!n) { // 完全二叉树必须后边所有的都是nullptr
                while (!q.empty()) {
                    auto x = q.front(); q.pop();
                    if (x) return false;
                }
                return true;
            }
            q.push(n->left);
            q.push(n->right);
        }
        return true;
    }
};