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852. Peak Index in a Mountain Array

Difficulty Topics

Description

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.

Solution

peak-index-in-a-mountain-array.py
class Solution:
    def peakIndexInMountainArray(self, arr: List[int]) -> int:
        low, high = 0, len(arr) - 1

        while low <= high:
            mid = low + (high - low) // 2

            if arr[mid] < arr[mid+1]:
                low = mid + 1

            elif arr[mid-1] > arr[mid]:
                high = mid - 1

            else:
                return mid

        return -1