# 55-I.二叉树的深度

### 一、 题目

输入一棵二叉树的根节点，求该树的深度。从根节点到叶节点依次经过的节点（含根、叶节点）形成树的一条路径，最长路径的长度为树的深度。

例如：

给定二叉树 \[3,9,20,null,null,15,7]，

```
     3
    / \
   9  20
     /  \
    15   7
```

返回它的最大深度 3 。

提示：

节点总数 <= 10000

来源：力扣（LeetCode）

链接：<https://leetcode-cn.com/problems/er-cha-shu-de-shen-du-lcof>

著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

### 二、 解法

#### 2.1 递归

```
func maxDepth(_ root: TreeNode?) -> Int {
    guard let tree = root else { return 0 }
    if tree.left == nil, tree.right == nil { return 1 }
    return max(maxDepth(tree.left), maxDepth(tree.right)) + 1
}
```

#### 2.2 非递归

```
func maxDepth(_ root: TreeNode?) -> Int {
    var deep = 0
    guard let root = root else { return deep }
    var queue: [TreeNode] = [root]
    var currentLevelCount = 1
    var nextLevelCount = 0
    
    while queue.isEmpty == false {
        let current = queue.removeFirst()
        currentLevelCount -= 1
        
        if let left = current.left {
            queue.append(left)
            nextLevelCount += 1
        }
        
        if let right = current.right {
            queue.append(right)
            nextLevelCount += 1
        }
        
        if currentLevelCount == 0 {
            deep += 1
            currentLevelCount = nextLevelCount
            nextLevelCount = 0
        }
    }
    
    return deep
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ryukiedev.gitbook.io/wiki/shu-ju-jie-gou-yu-suan-fa/jian-zhi-offerswift/55i.-er-cha-shu-de-shen-du.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.