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89 lines
2.6 KiB
Markdown
89 lines
2.6 KiB
Markdown
# [958. Check Completeness of a Binary Tree](https://leetcode.com/problems/check-completeness-of-a-binary-tree/)
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## 题目
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Given the `root` of a binary tree, determine if it is a *complete binary tree*.
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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`.
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**Example 1:**
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```
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Input: root = [1,2,3,4,5,6]
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Output: true
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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.
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```
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**Example 2:**
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```
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Input: root = [1,2,3,4,5,null,7]
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Output: false
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Explanation: The node with value 7 isn't as far left as possible.
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```
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**Constraints:**
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- The number of nodes in the tree is in the range `[1, 100]`.
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- `1 <= Node.val <= 1000`
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## 题目大意
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给定一个二叉树，确定它是否是一个完全二叉树。
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百度百科中对完全二叉树的定义如下：
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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。）
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## 解题思路
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- 这一题是按层序遍历的变种题。
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- 判断每个节点的左孩子是否为空。
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- 类似的题目，第 102，107，199 题都是按层序遍历的。
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## 代码
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```go
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package leetcode
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import (
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"github.com/halfrost/LeetCode-Go/structures"
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)
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// TreeNode define
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type TreeNode = structures.TreeNode
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/**
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* Definition for a binary tree node.
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* type TreeNode struct {
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* Val int
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* Left *TreeNode
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* Right *TreeNode
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* }
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*/
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func isCompleteTree(root *TreeNode) bool {
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queue, found := []*TreeNode{root}, false
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for len(queue) > 0 {
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node := queue[0] //取出每一层的第一个节点
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queue = queue[1:]
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if node == nil {
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found = true
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} else {
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if found {
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return false // 层序遍历中，两个不为空的节点中出现一个 nil
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}
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//如果左孩子为nil，则append进去的node.Left为nil
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queue = append(queue, node.Left, node.Right)
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}
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}
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return true
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}
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``` |