Range Sum Query - Immutable Solutions in C++

// Source : https://leetcode.com/problems/range-sum-query-immutable/
// Author : Calinescu Valentin, Hao Chen
// Date   : 2015-11-10



class NumArray {
/* 
 * Solution
 * =========
 *
 * The sum of all the elements starting from position 0 to position i is stored in
 * sums[i]. This way we can reconstruct the sum from position i to position j by
 * subtracting sums[i - 1] from sums[j], leaving us with the sum of the desired elements.
 *
 * Note: we can add a dummy sum at then beginning to simplify the code
 * 
 */
private:
    int size;
    vector <long long> sums;
public:
    NumArray(vector<int> &nums): size(nums.size()), sums(size+1, 0) {
        for(int i=0; i<size; i++) {
            sums[i+1] = sums[i] + nums[i];
        } 
    }
    int sumRange(int i, int j) {
        return sums[j+1] - sums[i];
    }
};


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

/// Source : https://leetcode.com/problems/range-sum-query-immutable/description/
/// Author : liuyubobobo
/// Time   : 2017-10-28

#include <iostream>
#include <vector>

using namespace std;

class NumArray {

private:
    vector<int> sum;

public:
    NumArray(vector<int> nums) {
        sum.push_back(0);
        for(int num: nums)
            sum.push_back(sum.back() + num);
    }

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

int main() {

    int nums[] = {-2, 0, 3, -5, 2, -1};
    vector<int> vec(nums, nums + sizeof(nums)/sizeof(int));
    NumArray obj(vec);

    cout << obj.sumRange(0, 2) << endl;
    cout << obj.sumRange(2, 5) << endl;
    cout << obj.sumRange(0, 5) << endl;

    return 0;
}