LeetCode.303.Range Sum Query - Immutable

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

Example:

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

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

Note:

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

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class NumArray {
public:
    NumArray(vector<int> &nums) {
        
       if (nums.empty())
            return ;
        else
        {
            sums.push_back(nums[0]);
            //求得给定数列长度
            int len = nums.size();
            for (int i = 1; i < len; ++i)
            {
                sums.push_back(sums[i - 1] + nums[i]);
            }//for
    }
    }

    int sumRange(int i, int j) {
       if (0 == i)
            return sums[j];
        int len = sums.size();

        if (i < 0 || i >= len || j < 0 || j >= len || i > j)
        {
            return 0;
        }//if
        return sums[j] - sums[i-1];
        
    }
   
private:
    vector<int> sums;
};