MountainArray.java

/*
 (This problem is an interactive problem.)

You may recall that an array arr is a mountain array if and only if:

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 mountainArr, return the minimum index such that mountainArr.get(index) == target. If such an index does not exist, return -1.

You cannot access the mountain array directly. You may only access the array using a MountainArray interface:

MountainArray.get(k) returns the element of the array at index k (0-indexed).
MountainArray.length() returns the length of the array.
Submissions making more than 100 calls to MountainArray.get will be judged Wrong Answer. Also, any solutions that attempt to circumvent the judge will result in disqualification.

 

Example 1:
Input: array = [1,2,3,4,5,3,1], target = 3
Output: 2
Explanation: 3 exists in the array, at index=2 and index=5. Return the minimum index, which is 2.

Example 2:
Input: array = [0,1,2,4,2,1], target = 3
Output: -1
Explanation: 3 does not exist in the array, so we return -1.
 

Constraints:

3 <= mountain_arr.length() <= 104
0 <= target <= 109
0 <= mountain_arr.get(index) <= 109
 */

 import java.util.*;

 class MountainArrayImpl{

    private int[] array;

    public MountainArrayImpl(int[] arr) {
        this.array = arr;
    }

    public int get(int index) {
        if (index < 0 || index >= array.length) {
            throw new IndexOutOfBoundsException("Invalid index: " + index);
        }
        return array[index];
    }

    public int length() {
        return array.length;
    }
}

public class MountainArray{

    private static int findInMountainArray(int target, MountainArrayImpl mountainArr) {
        int length = mountainArr.length();
        int peakIndex = findPeakIndex(1, length - 2, mountainArr);

        int increasingIndex = binarySearch(0, peakIndex, target, mountainArr, false);
        if (mountainArr.get(increasingIndex) == target) 
            return increasingIndex; 

        int decreasingIndex = binarySearch(peakIndex + 1, length - 1, target, mountainArr, true);
        if (mountainArr.get(decreasingIndex) == target) 
            return decreasingIndex; 

        return -1;  
    }

    private static int findPeakIndex(int low, int high, MountainArrayImpl mountainArr) {
        while (low != high) {
            int mid = low + (high - low) / 2;
            if (mountainArr.get(mid) < mountainArr.get(mid + 1)) {
                low = mid + 1; 
            } else {
                high = mid; 
            }
        }
        return low;
    }

    private static int binarySearch(int low, int high, int target, MountainArrayImpl mountainArr, boolean reversed) {
        while (low != high) {
            int mid = low + (high - low) / 2;
            if (reversed) {
                if (mountainArr.get(mid) > target)
                    low = mid + 1;
                else
                    high = mid;
            } else {
                if (mountainArr.get(mid) < target)
                    low = mid + 1;
                else
                    high = mid; 
            }
        }
        return low;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the number of elements: ");
        int n = sc.nextInt();
        System.out.println("Enter the elements: ");
        int[] arr = new int[n];
        for(int i = 0; i < n; i++){
              arr[i] = sc.nextInt();
        }
        System.out.println("Enter the target: ");
        int target = sc.nextInt();
        MountainArrayImpl mountainArr = new MountainArrayImpl(arr);
        System.out.println("The result is: " + findInMountainArray(target, mountainArr));
        sc.close();
    }
}