# [958. Check Completeness of a Binary Tree](https://leetcode.com/problems/check-completeness-of-a-binary-tree/)


## 题目

Given the `root` of a binary tree, determine if it is a *complete binary tree*.

In a **[complete binary tree](http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees)**, 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:**

![https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-1.png](https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-1.png)

```
Input: root = [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:**

![https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-2.png](https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-2.png)

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

```

**Constraints:**

- The number of nodes in the tree is in the range `[1, 100]`.
- `1 <= Node.val <= 1000`

## 题目大意

给定一个二叉树，确定它是否是一个完全二叉树。

百度百科中对完全二叉树的定义如下：

若设二叉树的深度为 h，除第 h 层外，其它各层 (1～h-1) 的结点数都达到最大个数，第 h 层所有的结点都连续集中在最左边，这就是完全二叉树。（注：第 h 层可能包含 1~ 2h 个节点。）

## 解题思路

- 这一题是按层序遍历的变种题。
- 判断每个节点的左孩子是否为空。
- 类似的题目，第 102，107，199 题都是按层序遍历的。

## 代码

```go
package leetcode

import (
	"github.com/halfrost/LeetCode-Go/structures"
)

// TreeNode define
type TreeNode = structures.TreeNode

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */

func isCompleteTree(root *TreeNode) bool {
	queue, found := []*TreeNode{root}, false
	for len(queue) > 0 {
		node := queue[0] //取出每一层的第一个节点
		queue = queue[1:]
		if node == nil {
			found = true
		} else {
			if found {
				return false // 层序遍历中，两个不为空的节点中出现一个 nil
			}
			//如果左孩子为nil，则append进去的node.Left为nil
			queue = append(queue, node.Left, node.Right)
		}
	}
	return true
}
```