Leetcode(958) Check Completeness of a Binary Tree

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Description

Given a binary tree, determine if it is a complete binary tree.

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

Example 1:

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Input: [1,2,3,4,5,6]
Output: true
Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.

Example 2:

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Input: [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.

Note:

  1. The tree will have between 1 and 100 nodes.

解法

按层遍历二叉树节点，无论当前节点的子节点是否为空均加入队列，如果在队列中碰到为空的节点，那么如果是一棵完全树，则队列中的剩余元素必都为空，否则是不完全的。

具体代码如下：

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/**
 * 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) {
        if(root == null){
            return true;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while(q.peek()!=null){
            TreeNode tmp = q.poll();
            q.offer(tmp.left);
            q.offer(tmp.right);
        }
        while(!q.isEmpty()){
            TreeNode tmp = q.poll();
            if(tmp != null){
                return false;
            }
        }
        return true;
    }
}