Program to Convert Infix expr to Postfix expression

import java.io.*;
import java.util.*;
import javax.swing.JOptionPane;
class Intopost
{
public static void main(String[] args)
{
String s=JOptionPane.showInputDialog("enter infix expression");
Stack st=new Stack();
String output="";
int i=0,len=s.length();
char x;
st.push('@');
while(len!=0)
{
char c=s.charAt(i);
if(c=='(')
{
st.push(c);
}
else if(c=='+'||c=='-'||c=='*'||c=='/'||c=='^'||c=='$')
{
check(st,c,output);
st.push(c);
}
else if(c==')')
{
while((x=(Character)st.pop())!='(')
{
output=output+x;
}
}
else
{
output+=s.charAt(i);
       }
 i=i+1;
 len--;
}
while((x=(Character)st.pop())!='@')
{
output+=x;
}
JOptionPane.showMessageDialog(null,output,"postfix expression",JOptionPane.INFORMATION_MESSAGE);
}
    static void check(Stack st,char c,String output)
{
while(priority(c)<=priority((Character)st.peek()))
output=output+st.pop();
}
static int priority(char ch)
{
if(ch=='+'||ch=='-')
return(1);
else if(ch=='*'||ch=='/')
return(2);
else if(ch=='$'||ch=='^')
return(3);
else
return(0);
}
}

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