852. Peak Index in a Mountain Array

Problem Link

Description

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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

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Python3

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