Arithmetic_413.java

public class Arithmetic_413 {
    // Recursion
    int sum = 0;

    public int numberOfArithmeticSlices(int[] A) {
        slices(A, A.length - 1);
        return sum;
    }

    private int slices(int[] A, int i) {
        if (i < 2)
            return 0;
        int ap = 0;
        if (A[i] - A[i - 1] == A[i - 1] - A[i - 2]) {
            ap = 1 + slices(A, i - 1);
            sum += ap;
        } else
            slices(A, i - 1);
        return ap;
    }
}

// DP looks better than Recursion (These approaches follow the same logic)
/*
public class Arithmetic_413 {
    public int numberOfArithmeticSlices(int[] A) {
        // DP (Time: O(n), Space: O(n))
        int[] dp = new int[A.length];
        int sum = 0;
        for (int i = 2; i < dp.length; i++) {
            if (A[i] - A[i - 1] == A[i - 1] - A[i - 2]) {
                dp[i] = 1 + dp[i - 1];
                sum += dp[i];
            }
        }
        return sum;


        // DP (Time: O(n), Space: O(1))
        int dp = 0;
        int sum = 0;
        for (int i = 2; i < A.length; i++) {
            if (A[i] - A[i - 1] == A[i - 1] - A[i - 2]) {
                dp = 1 + dp;
                sum += dp;
            } else
                dp = 0;
        }
        return sum;

    }
}
*/