Range Sum Query - Immutable

LeetCode Q 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.

Solution :

Since the array does not change and there are many calls to the sumRange function. Therefore, we consider to preprocess the array, calculating Sum(nums[1], ... nums[i]) and storing these values in an array.

Code:

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

public int sumRange(int i, int j) {
	return sum[j + 1] - sum[i];
}