sample.cpp

#include<iostream>
#include<cstring>
using namespace std;
struct node
{
	char data ;
	node *next ;
};
class stack
{
	private : 
	node *head;
	public :
	stack()
	{
		head = NULL;
	}
	void push(char a)
	{
		node * temp = new node();
		temp ->data = a ;
		temp -> next = head ; 
		head = temp ;
	}
	char pop()
	{
		node *temp = head ;
		head = temp->next ;
		char a = temp->data ;
		delete temp;
		return a;
	}
	char see_top()
	{
		if(is_empty())
			return '0' ;
		node * temp = head ;
		return (temp->data);
	}
	int is_empty()
	{
		if(head == NULL)
			return 1;
		else
			return 0 ;
	}
};
int is_allnum(char a)
{
	if(isalnum(a))
		return 1;
	else
		return 0;
}
int is_operand(char a)
{
	switch(a)
	{
		case '+' :
		case '-' :
		case '*' : 
		case '/' : return 1;
		default  : return 0;
	}
}
void reverse(char a[] , int l)
{
	int i=0;
	stack b ;
	for(int j=0;j<l;j++)
	{
		b.push(a[j]);
	}
	while(!b.is_empty())
		a[i++]=b.pop();
	return ;
}
	
int main()
{
	int i , l  , j=0;
	char exp[100] , infix[100];
	stack s ;
	cout << "Enter a postfix expression : ";
	cin.getline(exp,100);
	l=strlen(exp);
	reverse(exp,l);
	for(i=0;i<l;i++)
	{
		if(is_allnum(exp[i]))
		{
			infix[j++] = exp[i];
			if(i==l-1)
				break ;
			infix[j++] = s.pop() ;
		}
		else
			s.push(exp[i]);
	}
	while(!s.is_empty())
		infix[j++] =s.pop();
	reverse(infix,l);
	cout << "\nInfix is : " ;
	for(i=0;i<l;i++)
		cout << infix[i] ;
	cout << endl;
	return 0;
}