/**
 * Find the index in an array where sum to left is equal to sum to its right
 * @author PRATEEK
 */
class EquilibriumIndex {
 
	/**
	 * @param arr: input arr
	 * @return : equilibrium index in the array
	 */
	private int equilibrium(int[] arr) {
		int i = 0, sum = 0, rightSum = 0;
		for (; i < arr.length; sum += arr[i++]);
 
		for (i--; i > 0; sum -= arr[i], rightSum += arr[i])
			if (sum == rightSum - arr[i--])
				return i;
 
		return -1;
	}
 
	public static void main(String[] args) {
 
		int arr[] = { -7, 1, 6, 2, -4, 3, 1 };
		EquilibriumIndex obj = new EquilibriumIndex();
		int mid = obj.equilibrium(arr);
		System.out.println(mid);
 
	}
}
 

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