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 ` `#include ` `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 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; ` `} `

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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 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.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
```