Program to convet postfix int prefix

#include<stdio.h>

#include<conio.h>

#include<ctype.h>

#include<string.h>

#define MAX 20

char stack[MAX][MAX];

int top;

void posttopre(char post[30],char prefix[30])

{

      int top=-1,i;

      char x,st1[30];

      for(i=0;post[i]!='\0';i++)

      {

            x=post[i];

            if(isalnum(x))

            {

            st1[0]=x;

            st1[1]=NULL;

            top=top+1;

            strcpy(stack[top],st1);

            }

            else

            {

                  st1[0]=x;

                  st1[1]=NULL;

                  strcat(st1,stack(top-1));

                  strcat(st1,stack(top));

                  top=top-1;

                  strcpy(stack[top],st1);

                  }

            }

      }

       void main()

      {

            char post[30],prefix[30];

            char x;

            printf("\n enter the postfix expression=");

            gets(post);

            posttopre(post,prefix);

            strcpy(prefix,stack[top]);

            printf("\n\n the prefix expression is=5s",prefix);

            getch();

            }