Infix to Postfix Converter

The conversion from infix to postfix notation follows these rules:

1. Operands (numbers, variables) are directly added to the output.
2. For operators, we use a stack to maintain the correct order of operations.
3. When encountering an operator, we compare its precedence with operators on the stack.
4. Opening parentheses are pushed onto the stack and removed when a closing parenthesis is found.
5. After processing the entire expression, any remaining operators in the stack are popped to the output.