字符串减法（大数）

#include<iostream>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<map>
#include<set>
#define ll long long
using namespace std;
const int N = 1e6 + 10;

ll gcd(ll x, ll y) {
    return x % y == 0 ? y : gcd(y, x%y);
}
struct node {
    ll val;
    node* next;
};
int main()
{
    int t;
    cin >> t;
    while (t--) {
        string s, a, b = "";
        cin >> s;
        cin >> a;
        int len = s.length();
        
        //计算字符串s减去字符串a
        int len1 = a.length();
        int i = 0, j = 0, flag = 0;
        while (i < len&&j < len1) {
            s[i] = s[i] - flag;
            if (s[i] >= a[j]) {
                flag = 0;
                char temp = s[i] - a[j] + '0';
                b = b + temp;
            }
            else {
                flag = 1;
                char temp = 10 + s[i] - a[j] + '0';
                b = b + temp;
            }
            i++, j++;
        }
        while (i < len) {
            if (flag == 0)
                b = b + s[i];
            else {
                char temp = s[i] - 1;
                b = b + temp;
            }
            i++;
        }
        cout << b << endl;
    }
    system("pause");
    return 0;
}