Infix to Postfix

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

char st[100], post[100], s[100];
int top = -1, id = 0,i ;

void push(int x){
	top ++ ;
	st[top] = x ;
}
int pop (){
	return st[top--];
}
int operand(char x){
	return (x >= 'a' && x <= 'z') || (x >= 'A' && x <= 'Z');
}
int prec(char x){
	if(x == '*' || x == '/') return 2 ;
	else if(x == '+' || x == '-') return 1 ;
	return 0 ;
}

int main(){
	int n ;
	printf("Enter the infix expression : ");
	scanf("%s", s);
	n = strlen(s);
	push('(');
	s[n] = ')';

	for(i = 0 ; i <= n; i ++){
		if(operand(s[i])){
			post[id] = s[i];
			id ++ ;
		} else if(s[i] == '('){
			push(s[i]);
		} else if(s[i] != ')') {
			while(prec(st[top]) >= prec(s[i])){
				post[id ++] = pop();
			}
			push(s[i]);
		} else{
			while(st[top] != '('){
				post[id ++] = pop();
			}
			pop();
		}
	}

	printf("Posfix = %s", post);
	getch();
	return 0;
}