303. Range Sum Query - Immutable

Problem:

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:

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

Analysis:

Easy problem, use prefix sum.

Solution:

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