Arithmetic Slices II - Subsequence.java

/*
    A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference 
    between any two consecutive elements is the same.

    For example, these are arithmetic sequences:

    1, 3, 5, 7, 9
    7, 7, 7, 7
    3, -1, -5, -9
    The following sequence is not arithmetic.

    1, 1, 2, 5, 7

    A zero-indexed array A consisting of N numbers is given. A subsequence slice of that array is any sequence 
    of integers (P0, P1, ..., Pk) such that 0 ≤ P0 < P1 < ... < Pk < N.

    A subsequence slice (P0, P1, ..., Pk) of array A is called arithmetic if the sequence A[P0], A[P1], ..., 
    A[Pk-1], A[Pk] is arithmetic. In particular, this means that k ≥ 2.

    The function should return the number of arithmetic subsequence slices in the array A.

    The input contains N integers. Every integer is in the range of -231 and 2^{31}-1 and 0 ≤ N ≤ 1000. The output 
    is guaranteed to be less than 231-1.

    Link: https://leetcode.com/problems/arithmetic-slices-ii-subsequence/

    Example: 
        Input: [2, 4, 6, 8, 10]
        Output: 7
        Explanation:

        All arithmetic subsequence slices are:
        [2,4,6]
        [4,6,8]
        [6,8,10]
        [2,4,6,8]
        [4,6,8,10]
        [2,4,6,8,10]
        [2,6,10]

    Solution: None

    Source: https://discuss.leetcode.com/topic/67012/java-15-lines-solution/2
*/

public class Solution {
    public int numberOfArithmeticSlices(int[] A) {
        int re = 0;
        HashMap<Integer, Integer>[] maps = new HashMap[A.length];
        
        for (int i=0; i < A.length; i++) {
            maps[i] = new HashMap<>();
            int num = A[i];
            for (int j = 0; j < i; j++) {
                if ((long)num - A[j] > Integer.MAX_VALUE) {
                    continue;
                }
                if ((long)num - A[j] < Integer.MIN_VALUE) {
                    continue;
                }
                int diff = num - A[j];
                int count = maps[j].getOrDefault(diff, 0);
                maps[i].put(diff, maps[i].getOrDefault(diff, 0) + count + 1);
                re += count;
            }
        }
        return re;
    }
}