Postfix to Prefix Conversion

24 May, 2022
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) )
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) )Given a Postfix expression, convert it into a Prefix expression. Conversion of Postfix expression directly to Prefix without going through the process of converting them first to Infix and then to Prefix is much better in terms of computation and better understanding the expression (Computers evaluate using Postfix expression). 
Examples: 
Input :  Postfix : AB+CD-*
Output : Prefix :  *+AB-CD
Explanation : Postfix to Infix : (A+B) * (C-D)
              Infix to Prefix :  *+AB-CD

Input :  Postfix : ABC/-AK/L-*
Output : Prefix :  *-A/BC-/AKL
Explanation : Postfix to Infix : ((A-(B/C))*((A/K)-L))
              Infix to Prefix :  *-A/BC-/AKL 
Algorithm for Postfix to Prefix:  
Read the Postfix expression from left to right
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 before them. string = operator + operand2 + operand1 And push the resultant string back to Stack
Repeat the above steps until end of Postfix expression.
 Below is the implementation of the above idea:
C++
Java
Python3
C#
Javascript
// CPP Program to convert postfix to prefix#include <bits/stdc++.h>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 postfix to Prefix expressionstring postToPre(string post_exp){    stack<string> s;     // length of expression    int length = post_exp.size();     // reading from right to left    for (int i = 0; i < length; i++) {         // check if symbol is operator        if (isOperator(post_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 = post_exp[i] + op2 + op1;             // 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, post_exp[i]));        }    }     string ans = "";    while (!s.empty()) {        ans += s.top();        s.pop();    }    return ans;} // Driver Codeint main(){    string post_exp = "ABC/-AK/L-*";     // Function call    cout << "Prefix : " << postToPre(post_exp);    return 0;}
// Java Program to convert postfix to prefiximport 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 postfix to Prefix expression    static String postToPre(String post_exp)    {        Stack<String> s = new Stack<String>();         // length of expression        int length = post_exp.length();         // reading from right to left        for (int i = 0; i < length; i++) {             // check if symbol is operator            if (isOperator(post_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                    = post_exp.charAt(i) + op2 + op1;                 // Push String temp back to stack                s.push(temp);            }             // if symbol is an operand            else {                 // push the operand to the stack                s.push(post_exp.charAt(i) + "");            }        }         // concatenate all strings in stack and return the        // answer        String ans = "";        for (String i : s)            ans += i;        return ans;    }     // Driver Code    public static void main(String args[])    {        String post_exp = "ABC/-AK/L-*";         // Function call        System.out.println("Prefix : "                           + postToPre(post_exp));    }} // This code is contributed by Arnab Kundu
# Python3 Program to convert postfix to prefix # function to check if# character is operator or not  def isOperator(x):     if x == "+":        return True     if x == "-":        return True     if x == "/":        return True     if x == "*":        return True     return False # Convert postfix to Prefix expression  def postToPre(post_exp):     s = []     # length of expression    length = len(post_exp)     # reading from right to left    for i in range(length):         # check if symbol is operator        if (isOperator(post_exp[i])):             # pop two operands from stack            op1 = s[-1]            s.pop()            op2 = s[-1]            s.pop()             # concat the operands and operator            temp = post_exp[i] + op2 + op1             # Push string temp back to stack            s.append(temp)         # if symbol is an operand        else:             # push the operand to the stack            s.append(post_exp[i])         ans = ""    for i in s:        ans += i    return ans  # Driver Codeif __name__ == "__main__":     post_exp = "AB+CD-"         # Function call    print("Prefix : ", postToPre(post_exp)) # This code is contributed by AnkitRai01
// C# Program to convert postfix to prefixusing System;using System.Collections; 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 postfix to Prefix expression    static String postToPre(String post_exp)    {        Stack s = new Stack();         // length of expression        int length = post_exp.Length;         // reading from right to left        for (int i = 0; i < length; i++) {             // check if symbol is operator            if (isOperator(post_exp[i])) {                 // Pop two operands from stack                String op1 = (String)s.Peek();                s.Pop();                String op2 = (String)s.Peek();                s.Pop();                 // concat the operands and operator                String temp = post_exp[i] + op2 + op1;                 // Push String temp back to stack                s.Push(temp);            }             // if symbol is an operand            else {                 // Push the operand to the stack                s.Push(post_exp[i] + "");            }        }         String ans = "";        while (s.Count > 0)            ans += s.Pop();        return ans;    }     // Driver Code    public static void Main(String[] args)    {        String post_exp = "ABC/-AK/L-*";               // Function call        Console.WriteLine("Prefix : "                          + postToPre(post_exp));    }} // This code is contributed by Arnab Kundu
<script>    // Javascript Program to convert postfix to prefix         // function to check if character    // is operator or not    function isOperator(x)    {          switch (x) {        case '+':        case '-':        case '/':        case '*':            return true;        }        return false;    }      // Convert postfix to Prefix expression    function postToPre(post_exp)    {        let s = [];          // length of expression        let length = post_exp.length;          // reading from right to left        for (let i = 0; i < length; i++) {              // check if symbol is operator            if (isOperator(post_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 = post_exp[i] + op2 + op1;                  // Push String temp back to stack                s.push(temp);            }              // if symbol is an operand            else {                  // Push the operand to the stack                s.push(post_exp[i] + "");            }        }          let ans = "";        while (s.length > 0)            ans += s.pop();        return ans;    }         let post_exp = "ABC/-AK/L-*";            // Function call    document.write("Prefix : " + postToPre(post_exp));         // This code is contributed by suresh07.</script>
Prefix : *-A/BC-/AKL
Time Complexity: O(N) where N is the length of the string
Auxiliary Space: O(N) where N is the stack size.
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