Java program that converts infix expression into postfix form

import java.io.*;
class stack
{
	char stack1[]=new char[20];
	int top;
	void push(char ch)
	{
		top++;
		stack1[top]=ch;
	}
	char pop()
	{
		char ch;
		ch=stack1[top];
		top--;
		return ch;
	}
	int pre(char ch)
	{
		switch(ch)
		{
			case '-':return 1;
			case '+':return 1;
			case '*':return 2;
			case '/':return 2;
		}
		return 0;
	}
	boolean operator(char ch)
	{
		if(ch=='/'||ch=='*'||ch=='+'||ch=='-')
			return true;
		else
			return false;
	}
	boolean isAlpha(char ch)
	{
		if(ch>='a'&&ch<='z'||ch>='0'&&ch=='9')
			return true;
		else
			return false;
	}
	void postfix(String str)
	{
		char output[]=new char[str.length()];
		char ch;
		int p=0,i;
		for(i=0;i<str.length();i++)
		{
			ch=str.charAt(i);	
			if(ch=='(')
			{
				push(ch);
			}
			else if(isAlpha(ch))
			{
				output[p++]=ch;
			}
			else if(operator(ch))
			{
				if(stack1[top]==0||(pre(ch)>pre(stack1[top]))||stack1[top]=='(')
			{
				push(ch);
			}
			}
			else if(pre(ch)<=pre(stack1[top]))
			{
				output[p++]=pop();
				push(ch);
			}
			else if(ch=='(')
			{
				while((ch=pop())!='(')
				{
					output[p++]=ch;
				}
			}
		}
		while(top!=0)
		{
			output[p++]=pop();
		}
		for(int j=0;j<str.length();j++)
		{
			System.out.print(output[j]);	
		}
	}
}
class intopost
{
	public static void main(String[] args)throws Exception
	{
		String s;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		stack b=new stack();
		System.out.println("Enter input string");
		s=br.readLine();
		System.out.println("Input String:"+s);
		System.out.println("Output String:");
		b.postfix(s);
	}
}