打卡群刷题总结0825——括号生成

class Solution(object):
    def dfs(self, n):
        if n == 1:
            return 
        res_new = set()
        for ri in self.res:
            for j, rij in enumerate(ri):
                prefix = ri[:j + 1]
                suffix = ri[j + 1:] if j < len(ri) - 1 else ''
                res_new.update([prefix + '()' + suffix])
        self.res = res_new
        self.dfs(n - 1)
    
    def generateParenthesis(self, n):
        """
        :type n: int
        :rtype: List[str]
        """
        self.res = set(['()'])
        self.dfs(n)
        return self.res