input: Write a python function to evaluate a mathematical expression given in postfix notation.
  assert evaluate_expression("3 4 + 2 * 7 /") == 2
  assert evaluate_expression("5 1 2 + 4 * + 3 -") == 14
  assert evaluate_expression("10 2 8 * + 3 -") == 23
output: def evaluate_expression(expression):
      stack = []
      tokens = expression.split()
      for token in tokens:
          if token.isdigit():
              stack.append(int(token))
          else:
              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 == '/':
                  stack.append(a / b)
      return int(stack.pop())