中缀变后缀

import java.util.*;
public class Main {
    static String f(String str){//各子串按*/号断开
        StringBuilder r = new StringBuilder(String.valueOf(str.charAt(0)));
        for(int i = 1; i < str.length() - 1; i += 2){
            r.append(str.charAt(i + 1));//先压后面的操作数
            r.append(str.charAt(i));//再压中间的操作符
        }
        return String.valueOf(r);
    }
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        String[] v = s.split("[+-]");//原字符串按+-号断开
        StringBuilder r = new StringBuilder(f(v[0]));//压入第一个操作数
        int k = v[0].length();
        for(int i = 1; i < v.length; ++i){
            r.append(f(v[i]));//先压后面的操作数
            r.append(s.charAt(k));//再压中间的操作符
            k += v[i].length() + 1;
        }
        System.out.println(r);
    }
}