413. Arithmetic Slices

/**
 * Dynamic Programming
 *
 * validSlicesHere = Number of valid slices ending at the index i
 *
 * totalValidSlices = Total number of valid slices seen till now.
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(1)
 *
 * N = Length of the input array A.
 */
class Solution {
    public int numberOfArithmeticSlices(int[] A) {
        if (A == null || A.length < 3) {
            return 0;
        }

        int validSlicesHere = 0;
        int validSlicesSoFar = 0;

        for (int i = 2; i < A.length; i++) {
            // long is required to handle inout [Integer.MIN_VALUE, Integer.MAX_VALUE,
            // Integer.MAX_VALUE - 1]
            if ((long) A[i] - A[i - 1] == (long) A[i - 1] - A[i - 2]) {
                validSlicesHere++;
                validSlicesSoFar += validSlicesHere;
            } else {
                validSlicesHere = 0;
            }
        }

        return validSlicesSoFar;
    }
}