Range Sum Query - Immutable

Description

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.

Solution

sums[i]: the sum of first i numbers.

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

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