Thread: recursive string operation..

I need to build a function which gets two strings checks if their chars are alpha type
in ascending order.
If they both are then it needs to merge them in a third d string.

I cant call external functions.

The whole process must be in one function
and no iterative loops
and only in a recursive way.

I tried to build this function.

i think it works
but just after i input
abc in the first string and def into the second one

it does nothing and says that my program the EXE file
encountered a problem
and needs to be closed

why??

Code:

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res,index;
      int res2;
      //start checking apha chars for string 1
     if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
     {

         return 1;
     }
     if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
     {

         res=merge_strings(str1, index++,str2,index2, result, index3);
     }
     else
     {
       return 0;
     }
     //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
       {

         return 1;
     }
     if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
     {

         res2=merge_strings(str1, index,str2,index2++, result, index3);
     }
     else
     {
       return 0;
     }
     //end checking apha chars for string 2

     if ((res==1)&&(res2==1))
     {
         result[index3]=str1[index];
         if(index<255)
         {
             res=merge_strings(str1, index++,str2,index2, result, index3++);
         }

         result[index3]=str2[index2];
         if(index2<255)
         {
             res=merge_strings(str1, index,str2,index2++, result, index3++);
         }

     }
     return 1;
}