input: Write a function to evaluate a postfix expression given as a string. The function should return the evaluated value of the expression. If the input string is empty, it should return 0.
  def evaluate(expression): 
    
      if not expression: 
          return 0
    
      # split expression into list  
      expression = expression.split() 
    
      # stack to store integer values. 
      stack = []  
    
      # do for every element of expression. 
      for element in expression: 
    
          # If element is an operand push 
          # into stack it's a number only 
          if element.isdigit(): 
              stack.append(int(element)) 
    
          # if element is an operator, 
          # pop two elements from stack 
          # perform respective operations 
          else:  
              val1 = stack.pop() 
              val2 = stack.pop() 
              if element == '+': 
                  stack.append(val2 + val1) 
              elif element == '-': 
                  stack.append(val2 - val1) 
              elif element == '*': 
                  stack.append(val2 * val1) 
              elif element == '/': 
                  stack.append(val2 // val1) 
    
      # return the value  
      return stack[0] 
    
  assert evaluate("2 3 + 4 5 * *") == 46
  assert evaluate("2 3 + 5 *") == 25
  assert evaluate("10 2 8 * + 3 -") == 23
  assert evaluate("") == 0
  assert evaluate("3 4 +") == 7
output: def evaluate(expression): 
      if not expression: 
          return 0
    
      expression = expression.split() 
      stack = []  
    
      for element in expression: 
          if element.isdigit(): 
              stack.append(int(element)) 
          else:  
              val1 = stack.pop() 
              val2 = stack.pop() 
              if element == '+': 
                  stack.append(val2 + val1) 
              elif element == '-': 
                  stack.append(val2 - val1) 
              elif element == '*': 
                  stack.append(val2 * val1) 
              elif element == '/': 
                  stack.append(val2 // val1) 
    
      return stack[0]