Data Structure C program Prefix Conversion Dev c++

 #include <stdio.h>  
 #include <conio.h>  
 #define MAX 20  
 struct stack{  
 int top;  
 char str[MAX];  
 }s;//stack  
 char ex[MAX];//expression  
 char string[MAX];//string  
 int main(){  
   s.top=-1;  
   void push(char);  
   char pop();  
   int isoperand(char);  
   int getpriority(char);  
   printf("Enter Expression:");  
   scanf("%s",ex);  
   int i=strlen(ex)-1,j=-1,p1,p2;  
   while(i>=0)  
   {  
     if(isoperand(ex[i]))  
     {  
      printf("op\n");  
      string[++j]=ex[i];  
     }  
     else if(ex[i]==')')  
     {  
       push(')');  
       printf(")\n");  
     }  
     else if(ex[i]=='(')  
     {  
       printf("(\n");  
      while(s.str[s.top]!=')')  
       {  
        string[++j]=pop();  
        }  
        pop();  
     }else  
     {  
       p1=getpriority(ex[i]);  
       p2=getpriority(s.str[s.top]);  
       printf("%d<%d %d\n",p1,p2,p1<p2);  
       while(p1<p2 && s.top!=-1)  
       {  
        string[++j]=pop();  
        p2=getpriority(s.str[s.top]);  
       }  
       push(ex[i]);  
     }  
   i--;  
   }  
   while(s.top>-1)  
    string[++j]=pop();  
   printf("%s",strrev(string));  
   getch();  
 }  
 void push(char x)  
 {  
    if(s.top==MAX-1)  
     printf("Stack is full\nStack overflow\n");  
    else  
    {  
     s.top++;  
     s.str[s.top]=x;  
     printf("push %c\n",x);  
      getch();  
     }  
 }  
 char pop()  
 {  
    if(s.top==-1)  
    {  
          printf("Stack is emplty\nSTACK UNDERFLOW\n");  
          getch();  
    }  
    else  
    {  
      s.top--;  
      printf("pop %c\n",s.str[s.top+1]);  
      getch();  
      return s.str[s.top+1];  
    }  
 }  
 int isoperand(char x)  
 {  
   x=tolower(x);  
   if(x>='a' && x<='z')  
   return 1;  
   else  
   return 0;  
 }  
 int getpriority(char x){  
   if(x=='^')  
   return 3;  
   else if(x=='*' || x=='/')  
   return 2;  
   else if(x=='+' || x=='-')  
   return 1;  
   else   
   return 0;  
 }