22. Generate Parentheses——string——回溯

题目分析：有左括号可以随便加，右括号数要小于等于左括号

class Solution(object):
    def generateParenthesis(self, n):
        """
        :type n: int
        :rtype: List[str]
        """
        res = []
        self.generate(n, n, "", res)
        return res

    def generate(self, left, right, str, res):
        if left == 0 and right == 0:
            res.append(str)
            return
        if left > 0:
            # 左括号有剩余，可以加左括号
            self.generate(left - 1, right, str + '(', res)
        if right > left:
            # 剩余还没有用的右括号比左括号多
            self.generate(left, right - 1, str + ')', res)