PAT数字黑洞求大神解答

求大神看一下 ，测试了下第一次循环结尾num的值都是正确的 ，但进入第二次循环a和b的值就没了 导致程序就停止了

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
bool cmp(char a,char b)
{
    return a>b;
}
int tranfnum(string num)            //字符串转换为数字
{
    int sum = 0;
    int t = 1000;
    for(int i = 0;i < 4;i++)
    {
        int n = (num[i]-'0')*t;
        sum += n;
        t/=10;
    }
    return sum;
}
string tranchar(int num)            //数字转换为字符
{
    string result;
    int t = 1000;
    for(int i = 0;i < 4;i++)
    {
        if(num < t)
        {
            result[i] = '0';
            t/=10;
            continue;
        }
        result[i] = (num/t) + '0';
        num%=t;
        t/=10;
    }
    return result;

}
int main()
{
    std::ios::sync_with_stdio(false);
    string num,a,b;
    cin >> num;
    while(1)
    {
        a = num;b = num;
        sort(a.begin(),a.end());                //升序
        sort(b.begin(),b.end(),cmp);          //降序
        int frist = tranfnum(a);
        int sconed = tranfnum(b);
        int result = sconed - frist;
        if(result == 0)
        {
            cout << b << " - " << a << " = " << 0000 << endl;
            break;
        }
        cout << b << " - " << a << " = ";
        int t = 1000;
        int temp = result;
        while(temp < t)
        {
            cout << 0;
            t/=10;
        }
        cout << result << endl;
        if(result == 6174)
            break;
        num = tranchar(result);
    }
    return 0;
}

#include
#include
#include
#include
using namespace std;
//字符串转数字
int string_int(string num1)
{
int res1;
stringstream ss;
ss << num1;
ss >> res1;
return res1;
}
//数字转字符串
string int_string(int num)
{
string res;
stringstream ss;
ss << num;
ss >> res;
return res;
}
//字符串排序
bool cmp(char a, char b) { return a > b; }
int main()
{
string str, str1, str2, str3, str4;
cin >> str;
while (str.size() < 4)
{
str = "0" + str;//在str1的前面加0，补足4位；若在str1后面加0，应写为str1 = str1+“0”；
}
str1 = str;
str2 = str;
sort(str1.begin(), str1.end());
sort(str2.begin(), str2.end(), cmp);
while (str2 != str1 && string_int(str2) - string_int(str1) != 6174)
{
str4 = int_string(string_int(str2) - string_int(str1));
while (str4.size() < 4)
{
str4 = "0" + str4;
}
cout << str2 << " - " << str1 << " = " << str4 << endl;
str1 = str4;
str2 = str4;
sort(str1.begin(), str1.end());
sort(str2.begin(), str2.end(), cmp);
}
if (str2 == str1)
cout << str2 << " - " << str1 << " = " << "0000" << endl;
else
cout << str2 << " - " << str1 << " = " << 6174 << endl;
return 0;
}