Brace Expansion II

2019-11-26 11:05:10

• 1096. Brace Expansion II

Problem Description:

Problem Solving :

Classic string scaling issues.

In general there are two solutions of this problem, it is to use a stack, one is the use of recursive. Facts have proved that such a topic using recursive procedures in terms of time efficiency and readability is much higher than the stack, try using recursive.

This question has a very good explanation video: https://www.youtube.com/watch?v=blXuT7DOMwU

    public List<String> braceExpansionII(String expression) {
        List<String> res = new ArrayList<>();
        if (expression.length() <= 1) {
            res.add(expression);
            return res;
        }
        if (expression.charAt(0) == '{') {
            int cnt = 0;
            int idx = 0;
            for (; idx < expression.length(); idx++) {
                if (expression.charAt(idx) == '{') cnt += 1;
                if (expression.charAt(idx) == '}') cnt -= 1;
                if (cnt == 0) break;
            }
            List<String> strs = helper(expression.substring(1, idx));
            HashSet<String> set = new HashSet<>();
            for (String str : strs) {
                List<String> tmp = braceExpansionII(str);
                set.addAll(tmp);
            }
            List<String> rest = braceExpansionII(expression.substring(idx + 1));
            for (String str1 : set) {
                for (String str2 : rest) {
                    res.add(str1 + str2);
                }
            }
        }
        else {
            String prev = expression.charAt(0) + "";
            int idx = 0;
            List<String> rest = braceExpansionII(expression.substring(1));
            for (String s : rest) res.add(prev + s);
        }
        Collections.sort(res);
        return res;
    }
    
    public List<String> helper(String s) {
        List<String> res = new ArrayList<>();
        int cnt = 0;
        int i = 0;
        for (int j = 0; j < s.length(); j++) {
            if (s.charAt(j) == ',') {
                if (cnt == 0) {
                    res.add(s.substring(i, j));
                    i = j + 1;
                }
            }
            else if (s.charAt(j) == '{') cnt += 1;
            else if (s.charAt(j) == '}') cnt -= 1;
        }
        res.add(s.substring(i));
        return res;
    }