Mountain.java

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

public class Mountain
{
    public static void main(String[] args) {
        int arr[] = {0,5,10,2};
        int ans = peakIndexInMountainArray(arr);
        System.out.println(ans);
        
    }

    public static int peakIndexInMountainArray(int[] arr)
    {
        int start = 0;
        int end = arr.length - 1;
        while(start < end)
        {
            int mid = start + (end - start) / 2;
            if(arr[mid] > arr[mid + 1])
            {
                // you are in the descending part of the array 
                // this may be the ans but look at left side too 
                // this is why end != mid - 1
                end = mid;
            }
            else{
                // you are in the ascending part of the array
                start = mid + 1; // because we know that mid + 1 element > mid element
            }
        }
        // In the end ,start == end and pointing to the largest number because of the 2 checks above 
        // start and end are always trying to find max element in the above 2 checks 
        // hence , when they are pointing to just one element , that is the max one because that is what our checks say
        //more elaboration : at every point of time for start and end , they have the best possible answer till that time 
        // and if we are saying that only one item is remaining , hence because of above line that is the best possible answer
        return start; // or return end as both are =
    }
}