0199. Binary Tree Right Side View

# 199. Binary Tree Right Side View#

## 题目 #

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

Example:

``````
Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

1            <---
/   \
2     3         <---
\     \
5     4       <---

``````

## 解题思路 #

• 这一题是按层序遍历的变种题。按照层序把每层的元素都遍历出来，然后依次取每一层的最右边的元素即可。用一个队列即可实现。
• 第 102 题和第 107 题都是按层序遍历的。

## 代码 #

``````
package leetcode

/**
* Definition for a binary tree node.
* type TreeNode struct {
*     Val int
*     Left *TreeNode
*     Right *TreeNode
* }
*/
func rightSideView(root *TreeNode) []int {
if root == nil {
return []int{}
}
queue := []*TreeNode{}
queue = append(queue, root)
curNum, nextLevelNum, res, tmp := 1, 0, []int{}, []int{}
for len(queue) != 0 {
if curNum > 0 {
node := queue[0]
if node.Left != nil {
queue = append(queue, node.Left)
nextLevelNum++
}
if node.Right != nil {
queue = append(queue, node.Right)
nextLevelNum++
}
curNum--
tmp = append(tmp, node.Val)
queue = queue[1:]
}
if curNum == 0 {
res = append(res, tmp[len(tmp)-1])
curNum = nextLevelNum
nextLevelNum = 0
tmp = []int{}
}
}
return res
}

``````

Sep 6, 2020