303. Range Sum Query - Immutable -E

Q:

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.

A:

class NumArray {
public:
    NumArray(vector<int>& nums) {
        int sum = 0;
        for(auto v: nums)
        {
            sum += v;
            V.push_back(sum);
        }
    }
    
    int sumRange(int i, int j) {
        int pre = i>0? V[i-1] : 0;
        return V[j] - pre;
    }
private:
    vector<int> V; // assume no overflow on int 
};

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