Construct array with prefix sum takes `O(n)`

time.

sumRange takes constant time `O(1)`

.

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