ArrayEquilibriumIndex.java

// Array Equilibrium Index
// Send Feedback
// For a given array/list(ARR) of size 'N,' find and return the 'Equilibrium Index' of the array/list.
// Equilibrium Index of an array/list is an index 'i' such that the sum of elements at indices [0 to (i - 1)] is equal to the sum of elements at indices [(i + 1) to (N-1)]. One thing to note here is, the item at the index 'i' is not included in either part.
// If more than one equilibrium indices are present, then the index appearing first in left to right fashion should be returned. Negative one(-1) if no such index is present.
// Example:
// Let's consider an array/list Arr = [2, 3, 10, -10, 4, 2, 9]  of size, N = 7.

// There exist three equilibrium indices, one at 2, another at 3, and another at 5.

// At index 2, the sum of all the elements to the left, [2 + 3] is 5, and the elements to its right, [-10 + 4 + 2 + 9] is also 5. Hence index 2 is an equilibrium index according to the condition we want to achieve. Mind it that we haven't included the item at index 2, which is 10, to either of the parts.

// Similarly, we can see at index 3 and 5, the elements to its left sum up to 15 and 9 respectively and to the right, sum up to 15 and 9 respectively either. 

// Hence the answer would be 2.
// Input Format :
// The first line contains an Integer 't' which denotes the number of test cases or queries to be run. Then the test cases follow.

// The first line of each test case or query contains an integer 'N' representing the size of the first array/list.

// The second line contains 'N' single space separated integers representing the elements of the array/list
// Output Format :
// For each test case, print the 'Equilibrium Index'.

// Output for every test case will be printed in a separate line.
// Constraints :
// 1 <= t <= 10^2
// 0 <= N <= 10^6

// Time Limit: 1 sec 
// Sample Input 1 :
// 1
// 5
// 1 4 9 3 2
// Sample Output 1 :
// 2
// Sample Input 2 :
// 2
// 3
// 1 4 6
// 3
// 1 -1 4
// Sample Output 2 :
// -1
// 2

import java.util.Scanner;

public class ArrayEquilibriumIndex {
    public static void takeInput(int arr[], int n, Scanner s) {
        System.out.println("Enter Values Of The Array : ");
        for (int i = 0; i < n; i++) {
            arr[i] = s.nextInt();
        }
    }

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        System.out.print("Enter Number Of Test Cases : ");
        int t = s.nextInt();
        for (int i = 1; i <= t; i++) {
            System.out.print("Enter Length of " + i + "th Array : ");
            int n = s.nextInt();
            int arr[] = new int[n];
            takeInput(arr, n, s);
            int ans = arrayEquilibriumIndex(arr);
            System.out.println("Output : " + ans);
        }
        s.close();
    }

    public static int arrayEquilibriumIndex(int[] arr) {
        for (int i = 0; i < arr.length; i++) {
            boolean isTrue = check(arr, i);
            if (isTrue) {
                return i;
            }
        }
        return -1;
    }

    public static boolean check(int[] arr, int index) {
        int leftSum = 0;
        int rightSum = 0;
        for (int i = 0; i < index; i++) {
            leftSum += arr[i];
        }

        for (int i = index + 1; i < arr.length; i++) {
            rightSum += arr[i];
        }

        if (leftSum == rightSum) {
            return true;
        }

        return false;
    }
}