Prefix to Postfix Conversion

24 May, 2022
Prefix: An expression is called the prefix expression if the operator appears in the expression before the operands. Simply of the form (operator operand1 operand2). Example : *+AB-CD (Infix : (A+B) * (C-D) )
Postfix: An expression is called the postfix expression if the operator appears in the expression after the operands. Simply of the form (operand1 operand2 operator). Example : AB+CD-* (Infix : (A+B * (C-D) )Given a Prefix expression, convert it into a Postfix expression. Conversion of Prefix expression directly to Postfix without going through the process of converting them first to Infix and then to Postfix is much better in terms of computation and better understanding the expression (Computers evaluate using Postfix expression). 
Examples: 
Input :  Prefix :  *+AB-CD
Output : Postfix : AB+CD-*
Explanation : Prefix to Infix :  (A+B) * (C-D)
              Infix to Postfix :  AB+CD-*

Input :  Prefix :  *-A/BC-/AKL
Output : Postfix : ABC/-AK/L-*
Explanation : Prefix to Infix :  (A-(B/C))*((A/K)-L)
              Infix to Postfix : ABC/-AK/L-* 
Algorithm for Prefix to Postfix: 
Read the Prefix expression in reverse order (from right to left)
If the symbol is an operand, then push it onto the Stack
If the symbol is an operator, then pop two operands from the Stack Create a string by concatenating the two operands and the operator after them. string = operand1 + operand2 + operator And push the resultant string back to Stack
Repeat the above steps until end of Prefix expression.
C++
Java
Python 3
C#
Javascript
// CPP Program to convert prefix to postfix#include <iostream>#include <stack>using namespace std; // function to check if character is operator or notbool isOperator(char x){    switch (x) {    case '+':    case '-':    case '/':    case '*':        return true;    }    return false;} // Convert prefix to Postfix expressionstring preToPost(string pre_exp){     stack<string> s;    // length of expression    int length = pre_exp.size();     // reading from right to left    for (int i = length - 1; i >= 0; i--)    {        // check if symbol is operator        if (isOperator(pre_exp[i]))        {            // pop two operands from stack            string op1 = s.top();            s.pop();            string op2 = s.top();            s.pop();             // concat the operands and operator            string temp = op1 + op2 + pre_exp[i];             // Push string temp back to stack            s.push(temp);        }         // if symbol is an operand        else {             // push the operand to the stack            s.push(string(1, pre_exp[i]));        }    }     // stack contains only the Postfix expression    return s.top();} // Driver Codeint main(){    string pre_exp = "*-A/BC-/AKL";    cout << "Postfix : " << preToPost(pre_exp);    return 0;}
// JavaProgram to convert prefix to postfiximport java.util.*; class GFG {     // function to check if character    // is operator or not    static boolean isOperator(char x)    {        switch (x) {        case '+':        case '-':        case '/':        case '*':            return true;        }        return false;    }     // Convert prefix to Postfix expression    static String preToPost(String pre_exp)    {         Stack<String> s = new Stack<String>();         // length of expression        int length = pre_exp.length();         // reading from right to left        for (int i = length - 1; i >= 0; i--)        {            // check if symbol is operator            if (isOperator(pre_exp.charAt(i)))            {                // pop two operands from stack                String op1 = s.peek();                s.pop();                String op2 = s.peek();                s.pop();                 // concat the operands and operator                String temp = op1 + op2 + pre_exp.charAt(i);                 // Push String temp back to stack                s.push(temp);            }             // if symbol is an operand            else {                // push the operand to the stack                s.push(pre_exp.charAt(i) + "");            }        }         // stack contains only the Postfix expression        return s.peek();    }     // Driver Code    public static void main(String args[])    {        String pre_exp = "*-A/BC-/AKL";        System.out.println("Postfix : "                           + preToPost(pre_exp));    }} // This code is contributed by Arnab Kundu
# Write Python3 code here# -*- coding: utf-8 -*- # Example Inputs = "*-A/BC-/AKL" # Stack for storing operandsstack = [] operators = set(['+', '-', '*', '/', '^']) # Reversing the orders = s[::-1] # iterating through individual tokensfor i in s:     # if token is operator    if i in operators:         # pop 2 elements from stack        a = stack.pop()        b = stack.pop()         # concatenate them as operand1 +        # operand2 + operator        temp = a+b+i        stack.append(temp)     # else if operand    else:        stack.append(i) # printing final outputprint(*stack)
// C# Program to convert prefix to postfixusing System;using System.Collections.Generic; class GFG {     // function to check if character    // is operator or not    static bool isOperator(char x)    {        switch (x) {        case '+':        case '-':        case '/':        case '*':            return true;        }        return false;    }     // Convert prefix to Postfix expression    static String preToPost(String pre_exp)    {         Stack<String> s = new Stack<String>();         // length of expression        int length = pre_exp.Length;         // reading from right to left        for (int i = length - 1; i >= 0; i--)        {             // check if symbol is operator            if (isOperator(pre_exp[i]))            {                // pop two operands from stack                String op1 = s.Peek();                s.Pop();                String op2 = s.Peek();                s.Pop();                 // concat the operands and operator                String temp = op1 + op2 + pre_exp[i];                 // Push String temp back to stack                s.Push(temp);            }             // if symbol is an operand            else {                // push the operand to the stack                s.Push(pre_exp[i] + "");            }        }         // stack contains only the Postfix expression        return s.Peek();    }     // Driver Code    public static void Main(String[] args)    {        String pre_exp = "*-A/BC-/AKL";        Console.WriteLine("Postfix : "                          + preToPost(pre_exp));    }} /* This code contributed by PrinciRaj1992 */
<script>    // Javascript Program to convert prefix to postfix         // function to check if character    // is operator or not    function isOperator(x)    {        switch (x) {        case '+':        case '-':        case '/':        case '*':            return true;        }        return false;    }         // Convert prefix to Postfix expression    function preToPost(pre_exp)    {          let s = [];          // length of expression        let length = pre_exp.length;          // reading from right to left        for (let i = length - 1; i >= 0; i--)        {              // check if symbol is operator            if (isOperator(pre_exp[i]))            {                // pop two operands from stack                let op1 = s[s.length - 1];                s.pop();                let op2 = s[s.length - 1];                s.pop();                  // concat the operands and operator                let temp = op1 + op2 + pre_exp[i];                  // Push String temp back to stack                s.push(temp);            }              // if symbol is an operand            else {                // push the operand to the stack                s.push(pre_exp[i] + "");            }        }          // stack contains only the Postfix expression        return s[s.length - 1];    }         let pre_exp = "*-A/BC-/AKL";    document.write("Postfix : " + preToPost(pre_exp));         // This code is contributed by suresh07.</script>
Postfix : ABC/-AK/L-*
Time Complexity: O(N), as we are using a loop for traversing the expression.
Auxiliary Space: O(N), as we are using stack for extra space.
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