307. Range Sum Query - Mutable (Medium)

https://leetcode.com/problems/range-sum-query-mutable/

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

The update(i, val) function modifies nums by updating the element at index i to val.

Example:

Given nums = [1, 3, 5]

sumRange(0, 2) -> 9
update(1, 2)
sumRange(0, 2) -> 8

Note:

1. The array is only modifiable by the update function.
2. You may assume the number of calls to update and sumRange function is distributed evenly.

Solutions

class NumArray {
    int[] nums;
    int[] sums;

    public NumArray(int[] nums) {
        this.nums = nums;
        sums = new int[nums.length];

        if (nums.length == 0) {
            return;
        }

        for (int i = 0; i < nums.length; i++) {
            sums[i] = 0;
        }

        sums[0] = nums[0];
        for (int i = 1; i < nums.length; i++) {
            sums[i] = nums[i] + sums[i - 1];
        }
    }

    public void update(int i, int val) {
        if (nums.length == 0) {
            return;
        }

        // the subsequent sums will be affected
        int diff = nums[i] - val;
        for (int j = i; j < nums.length; j++) {
            sums[j] = sums[j] - diff;
        }

        nums[i] = val;
    }

    public int sumRange(int i, int j) {
        if (nums.length == 0) {
            return 0;
        }

        return sums[j] - sums[i] + nums[i];
    }
}

Incorrect Solutions

References