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website/content/ChapterFour/0637.Average-of-Levels-in-Binary-Tree.md

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+# [637. Average of Levels in Binary Tree](https://leetcode.com/problems/average-of-levels-in-binary-tree/)
+
+## 题目
+
+Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.
+
+**Example 1**:
+
+```
+
+Input:
+    3
+   / \
+  9  20
+    /  \
+   15   7
+Output: [3, 14.5, 11]
+Explanation:
+The average value of nodes on level 0 is 3,  on level 1 is 14.5, and on level 2 is 11. Hence return [3, 14.5, 11].
+
+```
+
+**Note**:  
+
+The range of node's value is in the range of 32-bit signed integer.
+ 
+
+## 题目大意
+
+按层序从上到下遍历一颗树，计算每一层的平均值。
+
+
+## 解题思路
+
+- 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 averageOfLevels(root *TreeNode) []float64 {
+	if root == nil {
+		return []float64{0}
+	}
+	queue := []*TreeNode{}
+	queue = append(queue, root)
+	curNum, nextLevelNum, res, count, sum := 1, 0, []float64{}, 1, 0
+	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--
+			sum += node.Val
+			queue = queue[1:]
+		}
+		if curNum == 0 {
+			res = append(res, float64(sum)/float64(count))
+			curNum, count, nextLevelNum, sum = nextLevelNum, nextLevelNum, 0, 0
+		}
+	}
+	return res
+}
+
+```