题目描述

Point:

1、公式

$$tmp = tmp + a[i] * b$$

2、注意前导0

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)
    {
        if(i < a.size()) t += a[i] * b;
        c.push_back(t % 10);
        t /= 10;
    }
    while(c.size() > 1 && c.back() == 0) c.pop_back();
    return c;
}
int main()
{
    string x;
    int b;
    cin >> x >> b;
    vector<int> a;

    for(int i = x.size() - 1; i >= 0; -- i) a.push_back(x[i] - '0');
    a = mul(a,b);
    for(int i = a.size() - 1; i >= 0; -- i) cout << a[i];
    return 0;
}