General steps:

- Find out all the arithmetic(a sequence of number); let's say its length is 'count';
- Using the formula arithmetic slices = (count-1)*(count-2)/2, we get all the possible arithmetic slices.
- Add them up, we will get the final result.

```
public class Solution {
public int numberOfArithmeticSlices(int[] A) {
if (A.length < 3) return 0;
int diff = A[1]-A[0];
int count = 2, res = 0;
for (int i = 2; i < A.length; i++) {
if (A[i]-A[i-1]==diff) {
count++;
}
else {
diff = A[i]-A[i-1];
if (count >=3) res+=(count-1)*(count-2)/2;
if (i<A.length-1 && A[i+1]-A[i]==diff) count = 2;
else if (i<A.length-1 && A[i+1]-A[i]!=diff) {
diff = A[i+1]-A[i];
count = 1;
}
}
}
if (count >=3) res+=(count-1)*(count-2)/2;
return res;
}
}
```

Time Complexity: O(N)

Space Complexity: O(1)

Runtime: 2ms beats 49.59%