# 1902. Depth of BST Given Insertion Order

https://leetcode.com/problems/depth-of-bst-given-insertion-order

## Description

You are given a **0-indexed** integer array `order` of length `n`, a **permutation** of integers from `1` to `n` representing the **order** of insertion into a **binary search tree**.

A binary search tree is defined as follows:

* The left subtree of a node contains only nodes with keys **less than** the node's key.
* The right subtree of a node contains only nodes with keys **greater than** the node's key.
* Both the left and right subtrees must also be binary search trees.

The binary search tree is constructed as follows:

* `order[0]` will be the **root** of the binary search tree.
* All subsequent elements are inserted as the **child** of **any** existing node such that the binary search tree properties hold.

Return _the **depth** of the binary search tree_.

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

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/06/15/1.png)

```

**Input:** order = [2,1,4,3]
**Output:** 3
**Explanation:** The binary search tree has a depth of 3 with path 2->3->4.
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2021/06/15/2.png)

```

**Input:** order = [2,1,3,4]
**Output:** 3
**Explanation:** The binary search tree has a depth of 3 with path 2->3->4.
```

**Example 3:**

![](https://assets.leetcode.com/uploads/2021/06/15/3.png)

```

**Input:** order = [1,2,3,4]
**Output:** 4
**Explanation:** The binary search tree has a depth of 4 with path 1->2->3->4.
```

**Constraints:**

* `n == order.length`
* `1 <= n <= 105`
* `order` is a permutation of integers between `1` and `n`.

## ac

```java
```
