ArithmeticSlices.java

package com.fishercoder.solutions;

/**A sequence of number 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 sequence:

 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 slice of that array is any pair of integers (P, Q) such that 0 <= P < Q < N.

 A slice (P, Q) of array A is called arithmetic if the sequence:
 A[P], A[p + 1], ..., A[Q - 1], A[Q] is arithmetic. In particular, this means that P + 1 < Q.

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


 Example:

 A = [1, 2, 3, 4]

 return: 3, for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] and [1, 2, 3, 4] itself.*/
public class ArithmeticSlices {

    //copied this solution: https://discuss.leetcode.com/topic/62884/2ms-java-o-n-time-o-1-space-solution
    public int numberOfArithmeticSlices(int[] A) {
        int sum = 0, len = 2;
        for (int i = 2; i < A.length; i++){
            if (A[i] - A[i-1] == A[i-1] - A[i-2]){
                len++;
            } else {
                if (len > 2) sum += calculateSlices(len);
                len = 2;//reset it to 2
            }
        }
        if(len > 2) sum += calculateSlices(len);
        return sum;
    }
    
    int calculateSlices(int len){
        return (len-1)*(len-2)/2;
    }
    
    class Solution2{
        //a more clear solution: https://discuss.leetcode.com/topic/63302/simple-java-solution-9-lines-2ms
        public int numberOfArithmeticSlices(int[] A) {
            int sum = 0, curr = 0;
            for (int i = 2; i < A.length; i++){
                if (A[i] - A[i-1] == A[i-1] - A[i-2]){
                    curr++;
                    sum += curr;
                } else {
                    curr = 0;
                }
            }
            return sum;
        }
    }

}