303. Range Sum Query - Immutable

1.描述

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

2.分析

3.代码

class NumArray {
public:
    NumArray(vector nums) {
        sum.push_back(0);
        for (int n : nums) {
            sum.push_back(sum.back() + n);
        }
    }
    
    int sumRange(int i, int j) {
        return sum[j + 1] - sum[i];
    }
private:
    vector sum;
};

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