LeetCode 303. Range Sum Query - Immutable

class NumArray {
private:
    vector<int> sums;
public:
    NumArray(vector<int>& nums) {
        int sum = 0;
        for (int num : nums)
            sums.emplace_back(sum += num);
    }
    int sumRange(int i, int j) {
        return i == 0 ? sums[j] : sums[j] - sums[i - 1];
    }
};