303. Range Sum Query - Immutable

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.
Example:
Given nums = [-2, 0, 3, -5, 2, -1]
sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Note:
You may assume that the array does not change.
There are many calls to sumRange function.

public class NumArray {

    int sum[] = null;
    
    public NumArray(int[] nums) {
        if(nums.length == 0){
            return;
        }    
        sum = new int[nums.length];
        sum[0] = nums[0];
        for(int i = 1; i