LeetCode 22. Generate Parentheses 回溯 backtrack

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

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Simple codes:

class Solution:
    def generateParenthesis(self, n: int) -> List[str]:
        res = []
        def backtrack(p, left, right):
            nonlocal res
            if (left > 0):
                backtrack(p + "(", left-1, right)
            if (left < right):
                backtrack(p + ")", left, right-1)
            if (right == 0):
                res.append(p)
        backtrack("", n, n)
        return res