input: Write a python function to convert a given string of mathematical expression from infix notation to postfix notation.
  assert infixToPostfix("A+B*C") == "ABC*+"
  assert infixToPostfix("A*(B+C)") == "ABC+*"
  assert infixToPostfix("A+B*C/D-E") == "ABC*D/+E-"
output: def infixToPostfix(expression):
      precedence = {'+': 1, '-': 1, '*': 2, '/': 2, '(': 0}
      output = []
      stack = []
      
      for char in expression:
          if char.isalnum():
              output.append(char)
          elif char == '(':
              stack.append(char)
          elif char == ')':
              while stack and stack[-1] != '(':
                  output.append(stack.pop())
              stack.pop()  # Pop the '(' from stack
          else:
              while stack and precedence[char] <= precedence[stack[-1]]:
                  output.append(stack.pop())
              stack.append(char)
      
      while stack:
          output.append(stack.pop())
      
      return ''.join(output)