Parse and flatten string, Java

Parse and flatten string

//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

//1、有点像欧洲那旮瘩的白人哥，“f{a,b}c{d,e}” => facd face fbcd fbce。要用他的api。followup是f{a{b},c}，有嵌套    
class Rextester
{  
    public static void main(String args[])
    {
        Solution sol = new Solution();
        System.out.println(sol.expand("f{a,b}c{d,e}x").toString());
        System.out.println(sol.expandHier("f{a{b,c},d}").toString());
        System.out.println(sol.expandHier("f{{b,c},d}x").toString());
        System.out.println(sol.expandHier("f{a{b,f{a,b}c{d,e}x},d}x").toString());
    }
}

class Solution {
    
    public List<String> expand(String s) {
        List<String> res = new ArrayList<>();
        if (s.length() == 0) return res;
        StringBuilder sb = new StringBuilder();
        res.add("");
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) != '{') sb.append(s.charAt(i));
            else {
                List<String> tmp = new ArrayList<>();
                List<String> tokens = new ArrayList<>();
                int j = findMatchAndSplit(s, i, tokens, '{', '}', ',');
                for (String token: tokens)
                    for (String prefix: res)
                        tmp.add(prefix + sb + token);
                i = j;
                res = tmp;
                sb.setLength(0);
            }
        }
        if (sb.length() > 0) {
            List<String> tmp = new ArrayList<>();
            for (String prefix: res) tmp.add(prefix + sb);
            res = tmp;
        }
        return res;
    }
    
    public int findMatchAndSplit(String s, int fromIndex, List<String> tokens, char open, char close, char split) {
        int count = 0; 
        for (int i = fromIndex, j = fromIndex + 1; i < s.length(); i++) {
            if (s.charAt(i) == open) count++;
            else if (s.charAt(i) == close) {
                count--;
                if (count == 0) {
                    tokens.add(s.substring(j, i));
                    return i;
                }
            } else if (s.charAt(i) == split && count == 1) {
                tokens.add(s.substring(j, i));
                j = i + 1;
            }
        }
        return -1;
    }
    
    public List<String> expandHier(String s) {
        List<String> res = new ArrayList<>();
        for (String ans: expand(s)) {
            if (ans.indexOf('{') == -1) res.add(ans);
            else res.addAll(expandHier(ans));
        }
        return res;
    }
}


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