# 852. Peak Index in a Mountain Array

Medium
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 a mountain array arr, return the index i such that arr[0] < arr[1] < ... < arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].
You must solve it in O(log(arr.length)) time complexity.
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
Constraints:
• 3 <= arr.length <= 105
• 0 <= arr[i] <= 106
• arr is guaranteed to be a mountain array.

### 解題

Runtime: 97 ms, faster than 95.21%
Memory Usage: 9.9 MB, less than 43.11%
func peakIndexInMountainArray(arr []int) int {
left := 0
right := len(arr) - 1
for left < right {
mid := ( left + right )/2
if arr[mid] < arr[mid+1] {
left = mid + 1
} else {
right = mid
}
}
return left
}