Range Sum Query - Immutable

This page explains Java solution to problem Range Sum Query - Immutable using Prefix array.

Problem Statement

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

Example 1:

Input: nums = [-2, 0, 3, -5, 2, -1]
Output:
sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Solution

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package com.vc.easy;

class RangeSumQueryImmutable {
    int[] prefixArr;
    public RangeSumQueryImmutable(int[] nums) {
        prefixArr = new int[nums.length + 1];
        for(int i = 0; i < nums.length; i++) {
            prefixArr[i + 1] = prefixArr[i] + nums[i];
        }
    }

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

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(i,j);
 */

Time Complexity

O(N) to generate prefix sum array Where
N is total number of elements in an input array
O(1) to get range sum

Space Complexity

O(N) Where
N is total number of elements in an input array