WAP for concatenation two strings without using string.h header file ?

 WAP for concatenation of two strings without using string.h header file?  
 #include<stdio.h>  
 #include<stdlib.h>  
 #include<string.h>  
 main()  
 {  
   int n,i,c,flag,j;  
   c=1;  
   char string[100],a[100],ch;  
   printf("enter a string");  
   scanf("%s",string);  
   printf("%s",string);  
   n=strlen(string);  
   printf("%d\n",n);  
   a[0]=string[0];  
   for(i=1;i<n;i++)  
 {  
    flag=1;  
    ch=string[i];  
    for(j=0;j<=i;j++)  
    {  
      if(a[j]==ch)  
      flag=0;  
    }              
   if(flag==1)  
  {  
   c++;  
   a[i]=string[i];  
  }  
 }  
 printf("%d\n",count);  
 system("pause");  
 }