Using an

`int[] sums`

to store the sums for each index. Then you can only`sums[j + 1] - sums[i]`

when you want to query a certain sum.

```
public class NumArray {
private int[] sums;
public NumArray(int[] nums) {
sums = new int[nums.length + 1];
sums[0] = 0;
for (int i = 1; i < nums.length + 1; i++) {
sums[i] = sums[i-1] + nums[i-1];
}
}
public int sumRange(int i, int j) {
return sums[j + 1] - sums[i];
}
}
```