Solution to the array equilibrium problem on Codility

Solution.java

class Solution {
  
    public int solution(int[] A) {
        // false inputs
        if (A == null || A.length == 0) {
            return -1;
        }
        
        // 1-length array
        if (A.length == 1) {
            return 0;
        }

        // 2-length array
        if (A.length == 2) {
            if (A[0] == 0) {
                return 1;
            } else if (A[1] == 0) {
                return 0;
            } else {
                return -1;
            }
        }

        // normal case
        long rightSide = 0;
        for (int i = 0; i < A.length; i++) {
            rightSide += A[i];
        }
        long leftSide = 0;
        for (int i = 0; i < A.length; i++) {
            if (leftSide == rightSide - A[i]) {
                return i;
            }
            leftSide += A[i];
            rightSide -= A[i];
        }
        return -1;
    }

}