这是题解

#include <iostream>
#include <string>
#include <algorithm>
#include <cstring>
using namespace std;
int main()
{
    string a,b,c;
    int as[1000001],bs[1000001],cs[1000001];
    memset(as,0,sizeof(as));
    memset(cs,0,sizeof(cs));
    memset(bs,0,sizeof(bs));
    cin >> a >> b;
    if((a.size() < b.size())|| ((a.size() == b.size())&& a < b))
    {
        swap(a,b);
        c += '-';
    }
    int al = a.size(),bl = b.size(),cl = max(a.size(),b.size());
    for(int i = 1; i <= al; i++) as[i] = a[al - i] - '0';
    for(int i = 1; i <= bl; i++) bs[i] = b[bl - i] - '0';
    for(int i = 1; i <= cl; i++) 
    {
        if(as[i] < bs[i])
        {
            as[i] += 10;
            as[i + 1]--;
        }
        cs[i] = as[i] - bs[i];
    }
    while(cl > 1 && cs[cl] == 0) cl--;
    for(int i = cl; i >= 1; i--) c += cs[i] + '0';
    cout << c;
    return 0;
}