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# Peak Index in a Mountain Array

Easy

[Peak Index in a Mountain Array 題目](https://leetcode.com/problems/peak-index-in-a-mountain-array/)

{% embed url="<https://leetcode.com/problems/peak-index-in-a-mountain-array/>" %}

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

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

Given an array that is definitely a mountain, return any `i` such that `A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]`.

**Example 1:**

```
Input: [0,1,0]
Output: 1
```

**Example 2:**

```
Input: [0,2,1,0]
Output: 1
```

**Note:**

1. `3 <= A.length <= 10000`
2. `0 <= A[i] <= 10^6`
3. A is a mountain, as defined above.