前缀和

https://oi-wiki.org/basic/prefix-sum/

class NumArray {
public:
    vector<int> prefixeSum = vector<int>(10001, 0);
    NumArray(vector<int>& nums) {
        int n = nums.size();
        for (int i = 0; i < n; ++i) {
            prefixeSum[i+1] = prefixeSum[i] + nums[i];
        }
    }

    /**
     * left 和 right 都是 nums 的索引, 从0到n-1
     * left <= i <= right, 属于闭区间
     */
    int sumRange(int left, int right) {
        return prefixeSum[right+1] - prefixeSum[left];
    }
};