[Solved] 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:

Question

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.

Python Solution

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

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

            if A[m-1]<A[m] and A[m+1]<A[m]:
                return (m)
            elif A[m-1]<=A[m] and A[m+1]>=A[m]:
                l=m+1
            else:
                r=m-1