Notes

import java.util.*;


public class NewClass1 {

private Stack<String> stack;
private String infixExp;
private String postfixExp = "";

public NewClass1(String exp){

String str = "";
infixExp = exp;
stack = new Stack<String>();

for (int i=0;i<infixExp.length();i++){
/*
* If the character is a letter or a digit we append it to the postfix
* expression directly.
*/
str = infixExp.substring(i,i+1);
if(str.matches("[a-zA-Z]|\d"))
postfixExp += str;
else if (isOperator(str)){
/*
* If the stack is empty we directly push the current char into it.
*/
if (stack.isEmpty()){
stack.push(str);
}
else{

String stackTop = stack.peek();
while (getPrecedence(stackTop,str).equals(stackTop)&& !(stack.isEmpty())){
postfixExp += stack.pop();
if (!(stack.isEmpty()))
stackTop = stack.peek();
}
stack.push(str);
}
}
}

while(!(stack.isEmpty()))
postfixExp += stack.pop();

System.out.println("Postfix: " + postfixExp);
}

/*
* Returns true if 'ch' is an operator, false o/w
*/
public boolean isOperator(String ch){

String operators = "*/%+-";
if (operators.indexOf(ch) != -1)
return true;
else
return false;
}


public String getPrecedence(String op1, String op2){

String multiplicativeOps = "*/%";
String additiveOps = "+-";

if ((multiplicativeOps.indexOf(op1) != -1) && (additiveOps.indexOf(op2) != -1))
return op1;
else if ((multiplicativeOps.indexOf(op2) != -1) && (additiveOps.indexOf(op1) != -1))
return op2;
else if((multiplicativeOps.indexOf(op1) != -1) && (multiplicativeOps.indexOf(op2) != -1))
return op1;
else
return op1;
}

public static void main(String[] args){
char ans;
Scanner console = new Scanner(System.in);


System.out.println("Enter infix:");
String expression = console.nextLine();
new NewClass1(expression);



}
}