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# 0852. Peak Index in a Mountain Array

<https://leetcode.com/problems/peak-index-in-a-mountain-array>

## Description

Let's call an array `arr` a **mountain** if the following properties hold:

* `arr.length >= 3`
* There exists some `i` with `0 < i < arr.length - 1` such that:
  * `arr[0] < arr[1] < ... arr[i-1] < arr[i]`
  * `arr[i] > arr[i+1] > ... > arr[arr.length - 1]`

Given an integer array `arr` that is **guaranteed** to be a mountain, return any `i` such that `arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1]`.

**Example 1:**

```
**Input:** arr = [0,1,0]
**Output:** 1
```

**Example 2:**

```
**Input:** arr = [0,2,1,0]
**Output:** 1
```

**Example 3:**

```
**Input:** arr = [0,10,5,2]
**Output:** 1
```

**Example 4:**

```
**Input:** arr = [3,4,5,1]
**Output:** 2
```

**Example 5:**

```
**Input:** arr = [24,69,100,99,79,78,67,36,26,19]
**Output:** 2
```

**Constraints:**

* `3 <= arr.length <= 104`
* `0 <= arr[i] <= 106`
* `arr` is **guaranteed** to be a mountain array.

**Follow up:** Finding the `O(n)` is straightforward, could you find an `O(log(n))` solution?

## ac

```java
```