LeetCode #1087. Brace Expansion

题目描述：

A string S represents a list of words.

Each letter in the word has 1 or more options.  If there is one option, the letter is represented as is.  If there is more than one option, then curly braces delimit the options.  For example, "{a,b,c}" represents options ["a", "b", "c"].

For example, "{a,b,c}d{e,f}" represents the list ["ade", "adf", "bde", "bdf", "cde", "cdf"].

Return all words that can be formed in this manner, in lexicographical order.

Example 1:

Input: "{a,b}c{d,e}f"
Output: ["acdf","acef","bcdf","bcef"]

Example 2:

Input: "abcd"
Output: ["abcd"]

Note:

1. 1 <= S.length <= 50
2. There are no nested curly brackets.
3. All characters inside a pair of consecutive opening and ending curly brackets are different.

class Solution {
public:
    vector expand(string S) {
        for(int i=0;i> v;
        istringstream iss(S);
        string tmp;
        while(getline(iss,tmp,' ')) 
        {
            if(tmp.empty()) continue;
            vector options;
            istringstream subiss(tmp);
            string subtmp;
            while(getline(subiss,subtmp,',')) 
                options.push_back(subtmp); 
            v.push_back(options);
        }

        vector result;
        DFS(v,0,"",result);
        sort(result.begin(),result.end());
        return result;
    }
    
    void DFS(vector>& v, int i, string cur, vector& result)
    {
        if(i==v.size())
        {
            result.push_back(cur);
            return;
        }
        for(string option:v[i]) DFS(v,i+1,cur+option,result);
    }
};