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.

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C++

1 class NumArray { 2 public: 3     NumArray(vector
     
       &nums) { 4         accu.push_back(0); 5         for (int num : nums) 6             accu.push_back(accu.back() + num); 7     } 8  9     int sumRange(int i, int j) {10         return accu[j + 1] - accu[i];11     }12 private:13     vector
      
        accu;14 };15 16 17 // Your NumArray object will be instantiated and called as such:18 // NumArray numArray(nums);19 // numArray.sumRange(0, 1);20 // numArray.sumRange(1, 2);
      
     

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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)