[LeetCode] Range Sum Query - Immutable

The idea is fairly straightforward: create an array accu that stores the accumulated sum fornums such that accu[i] = nums[0] + ... + nums[i] in the initializer of NumArray. Then just return accu[j + 1] - accu[i] in sumRange. You may try the example in the problem statement to convince yourself of this idea.

The code is as follows.

---

C++

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

    int sumRange(int i, int j) {
        return accu[j + 1] - accu[i];
    }
private:
    vector<int> accu;
};


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

---

Python

class NumArray(object):
    def __init__(self, nums):
        """
        initialize your data structure here.
        :type nums: List[int]
        """
        self.accu = [0]
        for num in nums:
            self.accu += self.accu[-1] + num,

    def sumRange(self, i, j):
        """
        sum of elements nums[i..j], inclusive.
        :type i: int
        :type j: int
        :rtype: int 
        """
        return self.accu[j + 1] - self.accu[i]


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