852. Peak Index in a Mountain Array

题目

You are given an integer mountain array arr of length n where the values increase to a peak element and then decrease.

Return the index of the peak element.

Your task is to solve it in O(log(n)) 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

Solution

class Solution:
    def peakIndexInMountainArray(self, arr: List[int]) -> int:
        length = len(arr)
        left = 1
        right = length - 2
        ans = 0

        while (left <= right):
            mid = (left + right) // 2
            if arr[mid] > arr[mid + 1]:
                ans = mid
                right = mid - 1
            else:
                left = mid + 1



        return ans