input: Write a python function to evaluate an arithmetic expression in reverse Polish notation (RPN).
  assert eval_expr('3 4 +') == 7
  assert eval_expr('3 4 *') == 12
  assert eval_expr('15 7 1 1 + - / 3 * 2 1 1 + + -') == 5
  assert eval_expr('5 1 2 + 4 * + 3 -') == 14
output: def eval_expr(expression):
      stack = []
      operators = {'+', '-', '*', '/'}
      
      for token in expression.split():
          if token not in operators:
              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(int(a / b))  # Ensure integer division
          
      return stack[0]