Solution.java

/**
 * 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 {
    private int[] nums;
    private int[] sums;

    public NumArray(int[] nums) {
        this.nums = nums;
        this.sums = new int[nums.length];
        for (int i = 0; i < nums.length; i++) {
            if (i == 0) {
                sums[i] = nums[i];
            } else {
                sums[i] = nums[i] + sums[i - 1]; 
            }
        }
    }

    public int sumRange(int i, int j) {
        if (i == 0) {
            return sums[j];
        }
        return sums[j] - sums[i - 1];
    }
}


// Your NumArray object will be instantiated and called as such:
// NumArray numArray = new NumArray(nums);
// numArray.sumRange(0, 1);
// numArray.sumRange(1, 2);