leetcode958. Check Completeness of a Binary Tree

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题目链接
题目：
思路：将所有的结点全部押入队列中，空也压入，每次判断队列的头如果队列头为空了则跳出循环，如果此后队列中还有非空元素则不是完全二叉树。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isCompleteTree(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(queue.peek() != null){
            TreeNode node = queue.poll();
            queue.offer(node.left);
            queue.offer(node.right);
        }
        while(!queue.isEmpty() && queue.peek() == null){
            queue.poll();
        }
        return queue.isEmpty();
    }
}