package me.ramswaroop.arrays;
/**
* Created by IntelliJ IDEA.
*
* @author: ramswaroop
* @date: 8/22/15
* @time: 11:56 AM
*/
public class EquilibriumIndex {
/**
* EQUILIBRIUM INDEX of an array is an index such that the sum of elements at lower
* indexes is equal to the sum of elements at higher indexes.
* <p/>
* For example, in an array A = {-7, 1, 5, 2, -4, 3, 0}
* <p/>
* 3 is an equilibrium index, because
* A[0] + A[1] + A[2] = A[4] + A[5] + A[6]
* <p/>
* 6 is also an equilibrium index, because sum of zero elements is zero,
* i.e., A[0] + A[1] + A[2] + A[3] + A[4] + A[5] = 0
* <p/>
* 7 is not an equilibrium index, because it is not a valid index of array A.
*
* @param a
* @return equilibrium index of array {@param a}.
*/
public static int getEquilibriumIndex(int[] a) {
int totalSum = 0, leftSum = 0;
for (int i = 0; i < a.length; i++) {
totalSum += a[i];
}
for (int i = 0; i < a.length; i++) {
totalSum -= a[i]; // totalSum now holds the right sum from ith index to end
if (leftSum == totalSum) {
return i; // left sum == right sum
}
leftSum += a[i]; // update left sum
}
return -1;
}
public static void main(String a[]) {
System.out.println(getEquilibriumIndex(new int[]{-7, 1, 5, 2, -4, 3, 0}));
System.out.println(getEquilibriumIndex(new int[]{-7, 1, 5, 0, 0, 0, 0, 1, 2, -4, 1, 3, 0}));
System.out.println(getEquilibriumIndex(new int[]{4, 5, 2, 1, 6, 7, 8, 0, 1}));
System.out.println(getEquilibriumIndex(new int[]{0}));
System.out.println(getEquilibriumIndex(new int[]{0, 0, 0}));
System.out.println(getEquilibriumIndex(new int[]{1, 1}));
System.out.println(getEquilibriumIndex(new int[]{1, 1, 1}));
}
}