算法1

C++ 代码

#include<iostream>
#include<vector>

using namespace std;

vector<int> mul(vector<int> &A, int b)
{
    vector<int> C;
    int t=0;
    for (int i = 0; i<A.size() || t; ++ i)//如果A的所有位数都与b相乘结束但是t中仍有值,则应该进位所以加入t的判断条件.
    {
        if (i < A.size() ) t += A[i] * b;//在保证A的仍然有位数时再与b相乘.
        //小细节:+=之间不能有空格,是一个整体.
        C.push_back(t % 10);
        t /= 10;
    }

    //去掉前导0.
    while (C.size() > 1 && C.back() == 0) C.pop_back();

    return C;
}

int main()
{
    string a;
    int b;

    cin>> a >> b;

    vector<int> A;
    for (int i = a.size() - 1;i >= 0 ; -- i) A.push_back( a[i] - '0' );

    auto C = mul(A,b);

    for (int i = C.size() - 1; i >= 0; -- i) printf("%d",C[i]);

    return 0;

}