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。学习判断括号有效的剪枝技巧。
• https://leetcode.com/problems/generate-parentheses/solution/

class Solution:
    def generateParenthesis(self, n: int) -> List[str]:
        if n == 0:
            return None
        
        result = []
        
        def helper(s = '', left = 0, right = 0):
            if len(s) == 2 * n:
                result.append(s)
                return
            
            if left < n:
                helper(s + '(', left + 1, right)
            
            if right < left:
                helper(s + ')', left, right + 1)
        
        helper()
        return result