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

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

Method:
the constructor of the array is the key because of too many calls to sumRange function.

C++:
class NumArray {
public:
NumArray(vector &nums):m(nums) {
for(int i=1; i m[i]+=m[i-1];
}
}

int sumRange(int i, int j) {
    if (i == 0) return m[j];
    return m[j]-m[i-1];
}

private:
vector m;
};

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