题解 | #判断是不是完全二叉树# 层序遍历

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return bool布尔型
     */
    public boolean isCompleteTree (TreeNode root) {
        // write code here
        Deque<TreeNode> que = new LinkedList<>();
        que.offer(root);
        boolean preNull = false;
        while (!que.isEmpty()) {
            int n = que.size();
            for (int i = 0; i < n; i++) {
                TreeNode current = que.poll();
                if (current == null) {
                    preNull = true;
                    continue;
                } 
                if (preNull) {
                    return false;
                }
                TreeNode left = current.left;
                TreeNode right = current.right;
                que.offer(left);
                que.offer(right);
            }
        }
        return true;
    }
}