Peak Index in a Mountain Array — Python

# Time:  O(logn)
# Space: O(1)

class Solution(object):
    def peakIndexInMountainArray(self, arr):
        """
        :type arr: List[int]
        :rtype: int
        """
        left, right = 0, len(arr)-1
        while left <= right:
            mid = left + (right-left)//2
            if arr[mid] > arr[mid+1]:
                right = mid-1
            else:
                left = mid+1
        return left