以Python为例实现反向波兰表达式

# -*- coding: utf-8 -*-
symbol_priority = {}
symbol_priority[0] = ['#']
symbol_priority[1] = ['(']
symbol_priority[2] = ['+', '-']
symbol_priority[3] = ['*', '/']
symbol_priority[4] = [')']
def comparePriority(symbol, RPN_stack, symbol_stack):
  '''Compare priority between two symbols'''
  global symbol_priority
  if len(symbol_stack) > 0:
    symbol_pop = symbol_stack.pop()
  else:
    return
  for list in symbol_priority.values():
    if (symbol in list) and (symbol_pop in list):
      '''same priority'''
      symbol_stack.append(symbol_pop)
      symbol_stack.append(symbol)
      return
    elif symbol in list:
      '''symbol is smaller'''
      RPN_stack.append(symbol_pop)
      #recusion call
      comparePriority(symbol, RPN_stack, symbol_stack)
      return
    elif symbol_pop in list:
      '''symbol is bigger'''
      symbol_stack.append(symbol_pop)
      symbol_stack.append(symbol)
      return
    else:
      continue
    symbol_stack.append(symbol_pop)
    return
def scanEveryone(input_string, RPN_stack, symbol_stack):
  for ch in input_string:
    if ch.isdigit():
      RPN_stack.append(ch)
    else:
      if len(symbol_stack) > 0:
        if ch == '(':
          symbol_stack.append(ch)
        elif ch == ')':
          while True:
            symbol_pop = symbol_stack.pop()
            if symbol_pop == '(':
              break
            else:
              RPN_stack.append(symbol_pop)
        else:
          comparePriority(ch, RPN_stack, symbol_stack)
      else:
        symbol_stack.append(ch)
def scanInput(RPN_stack, symbol_stack):
  input_string = raw_input()
  input_string += '#'
  scanEveryone(input_string, RPN_stack, symbol_stack)
def calRPN(RPN_stack):
  value_stack = []
  RPN_stack.append('#')
  for value in RPN_stack:
    if value == '#':
      return value_stack.pop()
      break
    if value.isdigit():
      value_stack.append(value)
    else:
      right_value = value_stack.pop()
      left_value = value_stack.pop()
      cal_string = left_value + value + right_value
      value_stack.append(str(eval(cal_string)))
def main():
  RPN_stack = []
  symbol_stack = []
 
  scanInput(RPN_stack, symbol_stack)
  print calRPN(RPN_stack)
if __name__ == '__main__':
  main()