Leetcode 22. Generate Parentheses

https://leetcode.com/problems/generate-parentheses/

Medium

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:

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

---

 

• 回溯，DFS。學習判斷括號有效的剪枝技巧。
• https://leetcode.com/problems/generate-parentheses/solution/

 1 class Solution:
 2     def generateParenthesis(self, n: int) -> List[str]:
 3         if n == 0:
 4             return None
 5         
 6         result = []
 7         
 8         def helper(s = '', left = 0, right = 0):
 9             if len(s) == 2 * n:
10                 result.append(s)
11                 return
12             
13             if left < n:
14                 helper(s + '(', left + 1, right)
15             
16             if right < left:
17                 helper(s + ')', left, right + 1)
18         
19         helper()
20         return result        

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