Objective: Given a Postfix expression, write an algorithm to convert it into prefix expression.
Input: Postfix expression: A B + Output: Prefix expression- + A B Input: Postfix expression: ABC/-AK/L-* Output: Infix expression: *-A/BC-/AKL
Approach: Use Stack
Iterate the given expression from left to right, one character at a time
- If the character is operand, push it to stack.
- If the character is operator,
- Pop operand from the stack, say it’s s1.
- Pop operand from the stack, say it’s s2.
- perform (operator s2 s1) and push it to stack.
- Once the expression iteration is completed, initialize the result string and pop out from the stack and add it to the result.
- Return the result.
Please walk through the example below for more understanding.
Postfix Expression: ABC/-AK/L-* Prefix Expression: *-A/BC-/AKL