ArithmeticSlices.java

package medium;

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;
        }
    }

}