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:

1. You may assume that the array does not change.

2. There are many calls to sumRange function.

class NumArray {
public:
    NumArray(vector<int> nums) { // time: O(n); space: O(n)
        sums.push_back(0);
        for (int num : nums) {
            sums.push_back(sums.back() + num);
        }
    }
    
    int sumRange(int i, int j) { // time: O(1)
        return sums[j + 1] - sums[i];
    }
    vector<int> sums; // sums[i]: accumulative sum for nums[0...i - 1]
};