We calculate the value of sum from 0 to position i right when we receive the table.

The value of sum from i to j is then:

num[j] - num[i-1] if i > 0

otherwise if i ==0 then we just need to return the position j in the table.

```
NumArray(vector<int> nums): num(nums) {
for (int i=1; i < num.size(); i++){
num[i] = num[i] + num[i-1];
}
}
int sumRange(int i, int j) {
if (i==0) return num[j];
return num[j]-num[i-1];
}
```