构建乘积数组

vector<int> multiply(const vector<int>& A) {
    int len = A.size();
    vector<int> B(len, 1);
    int res = 1;
    for(int i = 0;i < len;i++){
        B[i] *= res;
        res *= A[i];
    }
    res = 1;
    for(int i = len - 1;i >= 0;i--){
        B[i] *= res;
        res *= A[i];
    }
    return B;
}