python实现逆波兰计算简单方法，python实现波兰,这是一段与算法相关的py

python实现逆波兰计算简单方法，python实现波兰,这是一段与算法相关的py

这是一段与算法相关的python源码分享。就用python方法实现了逆波兰计算的简单方法。只输入一段表达式，从而计算出结果的应用。

# -*- 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)))#www.iplaypy.com                def main():    RPN_stack = []    symbol_stack = []        scanInput(RPN_stack, symbol_stack)    print calRPN(RPN_stack)if __name__ == '__main__':    main()            

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