Write a program using pointer to concate two strings

#include<stdio.h>

int main(){

 int i=0,j=0;

 char *str1,*str2,*str3;

 puts("Enter first string");

 gets(str1);

 puts("Enter second string");

 gets(str2);

 printf("Before concatenation the strings are\n");

 puts(str1);

 puts(str2);

 while(*str1){

     str3[i++]=*str1++;

 }

 while(*str2){

     str3[i++]=*str2++;

 }

 str3[i]='\0';

 printf("After concatenation the strings are\n");

 puts(str3);

 return 0;

}

Output:

Enter first String:HI

Enter Second String:HELLO
After Concatenation the strings are HI HELLO