LeetCode 852. Peak Index in a Mountain Array 二分

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.

Solution

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

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