22.括号生成

class Solution:
    def generateParenthesis(self, n: int) -> List[str]:
        ans = []
        def get_braces(i,j,braces):
            if i == n and j == n:
                ans.append(braces)
            if i < n:
                braces += '('
                get_braces(i+1,j,braces)
                braces = braces[:-1]
            if j < i:
                braces += ')'
                get_braces(i,j+1,braces)
                braces = braces[:-1]

        get_braces(0,0,'')

        return ans