PTA_Have fun with numbers(C++)

#include<iostream>
#include<cstring>
using namespace std;
int main()
{
    int z=0;
    char fir[20]="0";
    int sec[20]={0},thi[20]={0},fo[21]={0};
    int len=0;
    cin>>fir;
    len=strlen(fir);
    int len2=len;
    for(int i=0;i<len;i++) sec[i]=fir[i]-'0';
    int i=len-1;
    for(i;i>0;i--) 
    {
        thi[i]=2*sec[i]+z;
        z=thi[i]/10;
        thi[i]%=10;
    }
    int t=thi[i];
    
thi[i]=2*sec[i]+z;
    if(t>=10) 
    {
        len2++;
        thi[i]=2*sec[i]+z;
        fo[0]=thi[i]/10    ;
        thi[i]%=10;
        for(int j=0;j<len;j++) fo[j+1]=thi[j];
    }
    else 
    {
        thi[i]=2*sec[i]+z;
        for(int j=0;j<len2;j++) fo[j]=thi[j];
    }
        
    z=0;
    for(int k=0;k<len2;k++)
    {
        for(int j=0;j<len;j++)
        {
            if(sec[j]==fo[k]){sec[j]=100;z=11;break;}
        }
        if(z!=11) {z=0;break;}
        if(z==11) z=10;
        
    } 
    
    if(z==10)
    {
        cout<<"Yes"<<endl;
        for(int j=0;j<len2;j++) cout<<fo[j];
    }
    if(z!=10)
    {
        cout<<"No"<<endl;
        for(int j=0;j<len2;j++) cout<<fo[j];
    }
    return 0;
}