infix to postfix and infix to prefix conversion

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<string.h>

struct stack
{
char data;
struct stack *link;
};

struct stack *top=NULL;

void push(char c)
{
 struct stack *temp;
 temp=(struct stack *)malloc(sizeof(struct stack *));
 temp->data=c;
 temp->link=top;
 top=temp;
}

char pop()
{
 char data;
 data=top->data;
 top=top->link;
 return data;
}

char peek()
{
 return top->data;
}
int precedence(char c)
{
if(c=='^')
return 3;
if(c=='*' || c=='/')
return 2;
if(c=='+' || c=='-')
return 1;
else
return 0;
}


void infix2post(char infix[])
{
 int i=0;
 while(infix[i]!='')
 {
 //check operand --> send operand to output
  if((infix[i]>='a' && infix[i]<='z')||(infix[i]>='A' && infix[i]<='Z'))
  printf("%c",infix[i]);
  else if(infix[i]=='(')     //if ( push to stack
  push(infix[i]);
  else if(infix[i]==')')     // continously pop untill ) is encountered
  {
  while(peek()!='(')
  {
   printf("%c",pop());
  }
  pop();
  }
  else          // if operator is encountered
  {
   if(precedence(infix[i])>precedence(peek()))
   push(infix[i]);
   else
   {
   while(precedence(infix[i])<=precedence(peek()))
   printf("%c",pop());
   push(infix[i]);
   }
  }
  i++;
 }
}

void infix2prefix(char infix[])
{
int i=0,j;
char prefix[20];
char c;
 while(infix[i]!='')
 {
  c=infix[i++];
  if(c=='(')
  push(')');
  else if(c==')')
  push('(');
  else
  push(c);
 }
 i=0;
 j=0;
 while(top!=NULL)
 infix[i++]=pop();
 infix[i]='';
 i=0;
 while(infix[i]!='')
 {
 //check operand --> send operand to output
  if((infix[i]>='a' && infix[i]<='z')||(infix[i]>='A' && infix[i]<='Z'))
  prefix[j++]=infix[i];
  else if(infix[i]=='(')     //if ( push to stack
  push(infix[i]);
  else if(infix[i]==')')     // continously pop untill ) is encountered
  {
  while(peek()!='(')
  {
   prefix[j++]=pop();
  }
  pop();
  }
  else          // if operator is encountered
  {
   if(precedence(infix[i])>precedence(peek()))
   push(infix[i]);
   else
   {
   while(precedence(infix[i])<=precedence(peek()))
   prefix[j++]=pop();
   push(infix[i]);
   }
  }
  i++;
 }
 prefix[j]='';

 printf("%s",strrev(prefix));
}

void main()
{
 char expression[100];
 int choice;
 do
 {
 	printf("nn*********Menu************nn");
 	printf("Press 1 for Infix to Postfix conversionnn");
 	printf("press 2 for Infix to Prefix conversionnn");
 	printf("any other key to exitn");
 	scanf("%d",&choice);
 	switch(choice)
 	{
 		case 1:
 			printf("Enter an infix Epression : ");
 			scanf("%s",expression);
 			printf("Postfix Expression n");
 			infix2post(expression);
 			//printf("n Result after expression evaluation is : %d",infix2post(postfix));	
		break;

		case 2:
			printf("nEnter Infix Expressionn");
 			scanf("%s",expression);
 			printf("prefix Expression n");
 			infix2prefix(expression);
 			//printf("ninfix after conversion of prefix is: %d", infix2prefix(prefix));
		break;

		default:
			exit(0);
	}
 }while(choice);

getch();
}