# LeetCode 852. Peak Index in a Mountain Array

## Description

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 < A < ... 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 < A < ... 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.

binary search

## Python Solution

``````class Solution:
def peakIndexInMountainArray(self, A: List[int]) -> int:
if not A:
return -1

start = 0
end = len(A) - 1

while start + 1 < end:
mid = start + (end - start) // 2

if A[mid] > A[mid - 1] and A[mid] > A[mid + 1]:
return mid
elif A[mid] < A[mid + 1]:
start = mid
else:
end = mid

if A[end] > A[start]:
return end
else:
return start

return -1``````
• Time complexity: ~log(N)
• Space complexity: ~1