303. Range Sum Query - Immutable

class NumArray {
public:
    NumArray(vector<int> &nums) {
        int nums_size = nums.size();
        sum.resize(nums_size);
        for(int i=0;i<nums_size;i++){
            sum[i] = i==0 ? nums[i]:sum[i-1]+nums[i];
        }
    }

    int sumRange(int i, int j) {
        return i==0 ? sum[j] : sum[j] - sum[i-1];
    }
private:
    vector<int> sum;
};


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