Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

Example:

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Given nums = [-2, 0, 3, -5, 2, -1]

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Note:
You may assume that the array does not change.
There are many calls to sumRange function.

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class NumArray {
private:
    unordered_map<int, int> m;
public:
    NumArray(vector<int> nums) {
        m[-1] = 0;
        for (int i = 0; i < nums.size(); i++) {
            m[i] = m[i - 1] + nums[i];
        }
    }
    
    int sumRange(int i, int j) {
        return m[j] - m[i - 1];
    }
};

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(i,j);
 */