后缀表达式

分手后的思念是犯贱 2021-11-09 15:56 251阅读 0赞

package com.stack;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Stack;
    /**
     * @Auther: 大哥的叔
     * @Date: 2019/8/1 13:16
     * @Description:
     */
    public class PolandNotation {
        public static void main (String[] args) {
            //先定义后缀表达式
            //（3+4）*5-6=>3 4 + 5 * 6 -
            String suffixExpression = "3 4 + 5 * 6 -";
            //先将字符串放进数组
            //将数组传给一个方法
            List<String> rpnList = getListString(suffixExpression);
            System.out.println("rp"+rpnList);
            int res = calculate(rpnList);
            System.out.println(res);
        }
        //将后缀表达式存入数组
        public static List<String> getListString(String suffixExpression){
           //分割字符串
            String[] split = suffixExpression.split(" ");
            List<String> list = new ArrayList<String>();
            for (String ele:split
                 ) {
                list.add(ele);
            }
            return list;
        }
        //运算
        public static int calculate(List<String> ls){
        //创建栈
            Stack<String> stack = new Stack<String>();
            for (String item:ls
                 ) {
                if (item.matches("\\d+")){//\\d+表示[0—9]
                    stack.push(item);
                }else {
                    // pop出两个数，并运算， 再入栈
                    int num2 = Integer.parseInt(stack.pop());
                    int num1 = Integer.parseInt(stack.pop());
                    int res = 0;
                    if (item.equals("+")) {
                        res = num1 + num2;
                    } else if (item.equals("-")) {
                        res = num1 - num2;
                    } else if (item.equals("*")) {
                        res = num1 * num2;
                    } else if (item.equals("/")) {
                        res = num1 / num2;
                    } else {
                        throw new RuntimeException("运算符有误");
                    }
                    //把res 入栈
                    stack.push("" + res);
                }
            }
            return Integer.parseInt(stack.pop());
        }
    }