找规律，对于第i为有贡献的东西就是a，b下标和为i的数字的乘积。主要是得去模拟。

#include <bits/stdc++.h>using namespace std;
typedef long long ll;
typedef double db;
typedef long double ldb;
typedef pair<int, int> pii;
typedef pair<ll, ll> PII;
#define pb emplace_back
//#define int ll
#define all(a) a.begin(),a.end()
#define x first
#define y second
#define ps push_back
#define endl '\n'
#define IOS ios::sync_with_stdio(0),cin.tie(0),cout.tie(0)
#define ve1 vector<ll>
#define ve2 vector<ve1>
void solve();const int N = 1e6 + 10;
ve1 a,b,c;
ll ls1,ls2;signed main() {IOS;solve();return 0;
}void mul(ve1 a,ve1 b)
{for(int i = 0; i < ls1; ++ i){for(int j = 0; j < ls2; ++ j){c[i+j] += a[i] * b[j];//*********这里也是加号，无语字c[i+j + 1] += c[i+j]/10;//***************这里是加号，别在错了c[i+j] %= 10;}}
}void solve() {a.clear(),b.clear(),c.clear();string s1,s2;cin >> s1 >> s2;ls1 = s1.length();ls2 = s2.length();ll l = max(ls1,ls2)*2;c.resize(l,0);for(int i = ls1-1; ~ i; -- i) a.pb(s1[i]-'0');for(int i = ls2-1; ~ i; -- i) b.pb(s2[i]-'0');mul(a,b);l--;while(c[l]==0 && l) l--;for(int i = l; ~i; -- i)cout << c[i];cout << endl;
}