22. Generate Parentheses

配对构建问题可以考虑使用递归。

 public List<String> generateParenthesis(int n) {
        List<String> list = new ArrayList<String>();
        backtrack(list, "", 0, 0, n);
        return list;
    }
   
    public void backtrack(List<String> list, String str, int open, int close, int max){
       
        if(str.length() == max*2){
            list.add(str);
            return;
        }
       
        if(open < max) backtrack(list, str+"(", open+1, close, max);
        if(close < open)
            backtrack(list, str+")", open, close+1, max);
    }