input: Write a function to evaluate the value of a postfix expression. The function should take a string of the postfix expression as input and return the evaluated value of that expression. In case of invalid input, the function should return an error message. If the input string is empty, the function should return 0.
  assert evaluate("2 3 + 4 5 * *") == 46
  assert evaluate("5 3 * 2 +") == 17
  assert evaluate("8 4 /") == 2
  assert evaluate("10 2 5 * + 7 -") == 8
  assert evaluate("") == 0
output: def evaluate(postfix):
      if not postfix:
          return 0
      
      stack = []
      tokens = postfix.split()
      
      for token in tokens:
          if token.isdigit() or (token[0] == '-' and token[1:].isdigit()):
              stack.append(int(token))
          else:
              if len(stack) < 2:
                  return "Error: Invalid input"
              b = stack.pop()
              a = stack.pop()
              if token == '+':
                  stack.append(a + b)
              elif token == '-':
                  stack.append(a - b)
              elif token == '*':
                  stack.append(a * b)
              elif token == '/':
                  if b == 0:
                      return "Error: Division by zero"
                  stack.append(a // b)
              else:
                  return "Error: Invalid operator"
      
      if len(stack) == 1:
          return stack[0]
      else:
          return "Error: Invalid input"