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# 104. Maximum Depth of Binary Tree

## 題目 :

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

### Example :

Note

Example 1:

Input: root = [3,9,20,null,null,15,7] Output: 3

Example 2:

Input: root = [1,null,2] Output: 2

Example 3:

Input: root = [] Output: 0

Example 4:

Input: root = [0] Output: 1

#### 解題思路 :

• 這題的解法也屬於可以用queue和bfs概念做出來的題目
• 一樣是把每一個level的node當作一round，將node丟進queue裡，直到queue沒有東西了，代表已經到底了

### queue解法

• time complexity: O(n) , space complexity: O(n)

Runtime: 4 ms, faster than 90.28% of Go online submissions for Maximum Depth of Binary Tree.

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 `````` ``````/** * Definition for a binary tree node. * type TreeNode struct { * Val int * Left *TreeNode * Right *TreeNode * } */ func maxDepth(root *TreeNode) int { if root == nil { return 0 } var queue []*TreeNode queue = append(queue,root) var depth int for len(queue) != 0 { length := len(queue) for i := 0; i < length; i++ { node := queue[0] if node.Left != nil { queue = append(queue, node.Left) } if node.Right != nil { queue = append(queue, node.Right) } queue = queue[1:] } depth++ } return depth } ``````