前后缀表达式

前缀表达式

后缀表达式（逆波兰）

public class PolandNotation {
    public static void main(String[] args) {
        String suffixExpression = "3 4 + 5 * 6 -";
        List<String> list = getListString(suffixExpression);
        int res = calculate(list);
        System.out.println("计算的结果是："+ res);
    }
    //将表达式转换成ArrayList
    public static List<String> getListString(String suffixExpression){
        String[] s = suffixExpression.split(" ");
        List list = new ArrayList();
        for (String l : s){
            list.add(l);
        }
        return list;
    }
    //进行逆波兰计算
    public static int calculate(List<String> ls){
        Stack<String> stack =new Stack<String>();
        for (String item : ls){
            if (item.matches("\d+")){
                stack.push(item);
            }else {
                int num1 = Integer.parseInt(stack.pop());
                int num2 = Integer.parseInt(stack.pop());
                int res = 0;
                if (item.equals("+")){
                    res = num1 + num2;
                } else if (item.equals("-")){
                    res = num2 - num1;
                }else if (item.equals("*")){
                    res = num2 * num1;
                }else if (item.equals("/")){
                    res = num2 / num1;
                }else {
                    throw new RuntimeException("运算符有误");
                }
                stack.push(res + "");
            }
        }
        return Integer.parseInt(stack.pop());
    }
}