Mountain.java

public class Mountain {
    public static void main(String[] args) {

    }
    public int peakIndexInMountainArray(int[] arr){
        int start = 0;
        int end = arr.length-1;

        while (start< end){
            // find the  middle element
            // int  mid = (start + mid)/2;
            //might be possible that(start+end)
            int mid =start + (end - start) /2;
            if(arr[mid] > arr[mid+1]){
                // you are in dec part of array
                // this may be the ans, but look at left
                // this is why end!= mid -1
                end=mid;
            } else{
                // you are in asc part of array
                start = mid +1; // because we know that mid+1 element > mid elemen
            }
        }
        // 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 the 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 cause of above line that is the best possible ans
        return start; // or return end as both are =
    }
}