大神帮我看看一下我写的怎么回事？编译没错运行就错

#include "iostream.h"

void mian()
{
    int a=1,b=1,c=1;
    int num=0;
    for(;a<=5;a++)
    {
        for(;b<=5;b++)
        {
            if(b!=a)
            {
                for(;c<=5;c++)
            {
            
            
                if(c!=b&&c!=a)
                {
                    
                    
                        cout<<++num;
                        cout<<a<<b<<c;
                }
                if(num%3==0)
                    cout<<endl;
               
                }
            }
        }
    }
               

            
                    
        
                    
   
}