Java

import java.io.*;
import java.util.*;
public class InToPreToPos{
	private static Stack<Character> operatorStack = new Stack<Character> ();
	private static Stack<Comparable> operandStack = new Stack<Comparable> ();

	private static String toPostfix(String infix){
		StringTokenizer s = new StringTokenizer(infix);
		String symbol, postfix = "";
		while (s.hasMoreTokens()){
			symbol = s.nextToken();
			if (Character.isDigit(symbol.charAt(0))){
				postfix = postfix + " " + (Integer.parseInt(symbol));
			}else if (symbol.equals("(")){
				Character operator = new Character('(');
				operatorStack.push(operator);
			}else if (symbol.equals(")")){
				while (operatorStack.peek().charValue() != '('){
					postfix = postfix + " " + operatorStack.pop();
				}
				operatorStack.pop();
			}else{
				while (!operatorStack.empty() && !(operatorStack.peek()).equals("(") && prec(symbol.charAt(0)) <= prec(operatorStack.peek().charValue()))
					postfix = postfix + " " + operatorStack.pop();
				Character operator = new Character(symbol.charAt(0));
				operatorStack.push(operator);
				}
			}
		while (!operatorStack.empty())
			postfix = postfix + " " + operatorStack.pop();
		return postfix;
	}
	private static String toPrefix(String infix){
		StringTokenizer pre = new StringTokenizer(infix);
 
		String symbol1 , prefix= "";
		while(pre.hasMoreTokens()){
			symbol1 = pre.nextToken();
  
			if(Character.isDigit(symbol1.charAt(0))){
				prefix = prefix+ " " +(Integer.parseInt(symbol1));
			}else if(symbol1.equals("(")){
				Character operand = new Character('(');
				operandStack.push(operand);
			}else if(symbol1.equals(")")){
				while (((Character)operandStack.peek()).charValue()!= '('){
					prefix = prefix + " " + operandStack.pop();
				}
				operandStack.pop();
			}else{
				while(!operandStack.empty()&&!(operandStack.peek()).equals("(")
						&& prec(symbol1.charAt(0))<= prec(((Character)operandStack.peek()).charValue()))
     
					prefix = prefix + " "+ operandStack.pop();
				Character operand = new Character(symbol1.charAt(0));
				operandStack.push(operand);
				}
			}
			while(!operandStack.empty())
				prefix = prefix + " " +operandStack.pop();
			return prefix;
	}
 
	private static int prec(char x){
		if (x == '+' || x == '-'){
			return 1;
		}
		else if (x=='^' && x == '*' || x == '/' || x == '%'){
			return 2;
		}
		else{
			return 0;
			}
	}
	public static void main(String args[]) throws IOException{
		BufferedReader keyboard = new BufferedReader (new InputStreamReader(System.in));
		String infix;
		System.out.print("Input a infix: ");
		infix = keyboard.readLine();
		System.out.println("Expression in postfix:" + toPostfix(infix));
		System.out.println("Expression in prefix: " + toPrefix(infix));
 
	}
}