input: Write a python function to convert an infix expression to postfix notation.
  assert infix_to_postfix("2+3*5") == "235*+"
  assert infix_to_postfix("9-4*2+7") == "942*-7+"
  assert infix_to_postfix("(5+2)*3") == "52+3*"
output: def infix_to_postfix(expression):
      precedence = {'+': 1, '-': 1, '*': 2, '/': 2, '^': 3}
      output = []
      operators = []
  
      for char in expression:
          if char.isnumeric():
              output.append(char)
          elif char == '(':
              operators.append(char)
          elif char == ')':
              while operators and operators[-1] != '(':
                  output.append(operators.pop())
              operators.pop()
          else:
              while operators and operators[-1] != '(' and precedence[char] <= precedence[operators[-1]]:
                  output.append(operators.pop())
              operators.append(char)
      
      while operators:
          output.append(operators.pop())
  
      return ''.join(output)