输入中缀表达式，转换为后缀表达式，并计算出结果-Input infix expression into postfix expression, and calculate the results

四则运算表达式求值，将四则运算表达式用中缀表达式，然后转换为后缀表达式，并计算结果。-Four arithmetic expression uation, the four arithmetic expressions used in infix expression, and then converted to a postfix expression, and calculate the results.