[leetcode] 22. Generate Parentheses

class Solution:
    def generateParenthesis(self, n: int) -> List[str]:
        '''
        type n = int
        
        '''
        self.res = []
        self.helper("",n,n)
        return self.res
    
    def helper(self, current,left, right) :
        if right == 0:
            self.res.append(current)
        else : 
            if left > 0:
                self.helper(current + "(", left-1, right)
            if left < right : 
                self.helper(current + ")", left, right-1)