303. Range Sum Query - Immutable

public class NumArray {

    private final int[] preSumArray;

    public NumArray(int[] nums) {
        this.preSumArray = new int[nums.length + 1];
        for (int i = 1; i < preSumArray.length; i++) {
            preSumArray[i] = preSumArray[i - 1] + nums[i - 1];
        }
    }

    public int sumRange(int left, int right) {
        return preSumArray[right + 1] - preSumArray[left];
    }

}

Reference

303. Range Sum Query - Immutable