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website/content/ChapterFour/0199.Binary-Tree-Right-Side-View.md

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+# [199. Binary Tree Right Side View](https://leetcode.com/problems/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       <---
+
+```
+
+ 
+
+## 题目大意
+
+从右边看一个树，输出看到的数字。注意有遮挡。
+
+
+## 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
+
+```go
+
+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
+}
+
+```