系统可随机给出4个数，也可由用户自行输入四张牌的值。用户输入表达式，先判断表达式是否正确，如果正确，计算是否24，不正确的话，给出可能的正确表达式。用户可放弃输入，由系统直接给出可能的答案-The system can randomly given four numbers, but also by the user to enter a value of four cards. User input expression, first determine the expression is c