0852-peak-index-in-a-mountain-array¶

Description¶

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(Python)¶

class Solution:
    def peakIndexInMountainArray(self, arr: List[int]) -> int:
        n = len(arr)
        l = 0
        r = n-1

        while l < r:
            m = (l + r) // 2

            if arr[m] > arr[m+1]:
                r = m
            else:
                l = m + 1

        return l
        