It needs a little math. 3 elements: 1 arithmetic, 4: 1+2=3 arithmetic, 5:1+2+3=6 arithmetic, n:len*(len+1)/2 . (len=n-2).

Two if(len>=1) condition checks can also be removed to make codes more concise. But I would rather keep them to make logic clear.

```
public int numberOfArithmeticSlices(int[] A) {
if(A==null || A.length<=2) return 0;
int count=0, len=0;
for(int i=2; i<A.length; i++){
if(A[i]-A[i-1]==A[i-1]-A[i-2]) len++;
else{
if(len>=1){
count+=len*(len+1)/2;
}
len=0;
}
}
if(len>=1) count+=len*(len+1)/2;
return count;
}
```