Java: Find the equilibrium indices from a given array of integers

Last update on May 09 2025 12:51:25 (UTC/GMT +8 hours)

62. Find equilibrium indices in an array

Write a Java program to find equilibrium indices in a given array of integers.

An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices.

For example, in a sequence A:
  A0 = -7
  A1 = 1
  A2 = 5
  A3 = 2
  A4 = -4
  A5 = 3
  A6 = 0
3 is an equilibrium index, because:

  A0 + A1 + A2 = A4 + A5 + A6
  
6 is also an equilibrium index, because:

  A0 + A1 + A2 + A3 + A4 + A5 = 0
  
(sum of zero elements is zero)

7 is not an equilibrium index, because it is not a valid index of sequence A.

Example:
Input :
nums = {-7, 1, 5, 2, -4, 3, 0}
Output:
Equilibrium indices found at : 3
Equilibrium indices found at : 6
Source: https://bit.ly/2ziUROQ

Sample Solution:

Java Code:

//Source: https://bit.ly/2ziUROQ

// Import the necessary Java class.
import java.util.Arrays;

// Define a class named 'solution'.
public class solution {
    public static void main(String[] args) {
        // Initialize an array of integers.
        int[] nums = {-7, 1, 5, 2, -4, 3, 0};
        System.out.println("Original array: " + Arrays.toString(nums));
        
        // Call the 'equlibrium_indices' method to find equilibrium indices.
        equlibrium_indices(nums);
    }

    public static void equlibrium_indices(int[] nums) {
        // Find the total sum of elements in the array.
        int totalSum = 0;
        for (int n : nums) {
            totalSum += n;
        }

        // Initialize a running sum to keep track of the sum as we iterate.
        int runningSum = 0;

        // Iterate through the array to find equilibrium indices.
        for (int i = 0; i < nums.length; i++) {
            int n = nums[i];

            // Check if the current index is an equilibrium index.
            if (totalSum - runningSum - n == runningSum) {
                System.out.println("Equilibrium indices found at: " + i);
            }

            // Update the running sum as we move through the array.
            runningSum += n;
        }
    }
}

Sample Output:

Original array: [-7, 1, 5, 2, -4, 3, 0]
Equilibrium indices found at : 3
Equilibrium indices found at : 6

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Java program to find all subarrays whose sum is equal to the sum of the remaining elements in the array.
• Write a Java program to determine if an array can be split into two parts with equal sum.
• Write a Java program to find an index such that the product of elements on the left equals the product of elements on the right.
• Write a Java program to find the maximum number of equilibrium indices in a given array.

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