BS_Peak_Mountain_Array.java

package dsa;
// https://leetcode.com/problems/peak-index-in-a-mountain-array/
// https://leetcode.com/problems/find-peak-element/

public class BS_Peak_Mountain_Array {
    public static void main(String[] args) {
       int[]arr={0,1,5,6,10,9,7,3,2};
       //int ans=peakIndexInMountainArray(arr);
        //System.out.println(ans);
    }
    public int peakIndexInMountainArray(int[]arr){
        int start=0;
        int end= arr.length-1;
        while (start<end){
            //int mid=start+end/2; // might be possible that (start + end) exceeds the range of int in java
            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 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 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 cuz of above line that is the best possible ans
        return start; // or return end as both are =
    }
}