Postfix to Prefix Conversion

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

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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 Prefix expression.

C++

// CPP Program to convert postfix to prefix 
#include <iostream> 
#include <stack> 
using namespace std; 
  
// function to check if character is operator or not 
bool isOperator(char x) 
{ 
    switch (x) { 
    case '+': 
    case '-': 
    case '/': 
    case '*': 
        return true; 
    } 
    return false; 
} 
  
// Convert postfix to Prefix expression 
string 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])); 
        } 
    } 
  
    // stack[0] contains the Prefix expression 
    return s.top(); 
} 
  
// Driver Code 
int main() 
{ 
    string post_exp = "ABC/-AK/L-*"; 
    cout << "Prefix : " << postToPre(post_exp); 
    return 0; 
} 

/div>

Java

// Java Program to convert postfix to prefix 
import 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) + ""); 
            } 
        } 
  
        // stack[0] contains the Prefix expression 
        return s.peek(); 
    } 
  
    // Driver Code 
    public static void main(String args[]) 
    { 
        String post_exp = "ABC/-AK/L-*"; 
        System.out.println("Prefix : " + postToPre(post_exp)); 
    } 
} 
  
// This code is contributed by Arnab Kundu 

C#

// C# Program to convert postfix to prefix 
using 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] + ""); 
            } 
        } 
  
        // stack[0] contains the Prefix expression 
        return (String)s.Peek(); 
    } 
  
    // Driver Code 
    public static void Main(String[] args) 
    { 
        String post_exp = "ABC/-AK/L-*"; 
        Console.WriteLine("Prefix : " + postToPre(post_exp)); 
    } 
} 
  
// This code is contributed by Arnab Kundu 

Output:

Prefix : *-A/BC-/AKL

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

C#

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