题解 | 牛牛与后缀表达式

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 给定一个后缀表达式，返回它的结果
     * @param str string字符串 
     * @return long长整型
     */
    public long legalExp (String str) {
        // write code here
        // long sum = 0;
        Stack<Long> zhan = new Stack<Long>();
        StringBuilder strb = new StringBuilder( ); 
        for(int i = 0; i<str.length(); i++){
           Character charc = str.charAt(i);
           if(Character.isDigit(charc)){
                strb.append(charc);
               
           }
           else if(charc.equals('#')){
                System.out.println("00:"+strb.toString());
                zhan.add(Long.parseLong(strb.toString()));
                strb.delete(0,strb.length());
                // System.out.println("11:"+strb.toString());
           }
           else if(charc.equals('+')){
                long sum = zhan.pop() + zhan.pop();
                zhan.push(sum);
                System.out.println("+:"+zhan.peek());
           }
          else if(charc.equals('-')){
                long sum  = -zhan.pop() + zhan.pop();
                 zhan.push(sum);
                  System.out.println("-:"+zhan.peek());
           }
          else if(charc.equals('*')){
                long sum  = zhan.pop() * zhan.pop();
                 zhan.push(sum);
                  System.out.println("*:"+zhan.peek());
           }
        }
        return zhan.pop();
    }
}