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 correct, if correct, the calculation is 24, is not correct, given possible correct expression. Users may waive input given directly by the system may answer