[Leet Code] Peak Index in a Mountain Array

Leetcode: https://leetcode.com/problems/peak-index-in-a-mountain-array/

Problem:

Let’s call 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 an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

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

Example 4:

Input: arr = [3,4,5,1]
Output: 2

Example 5:

Input: arr = [24,69,100,99,79,78,67,36,26,19]
Output: 2

Solution:

class Solution(object):
    def peakIndexInMountainArray(self, arr):
        """
        :type arr: List[int]
        :rtype: int
        """
        start = 0
        end = len(arr) - 1
        maxIndex = 0
        while (start <= end):
            mid = ((start + end) // 2)
            if arr[mid-1] < arr[mid] > arr[mid+1]:
                return mid
            elif arr[mid] < arr[mid+1]:
                start = mid + 1
            else:
                end = mid - 1

Example in written form substituting variables: