[leetcode]Generate Parentheses @ Python

原题地址：https://oj.leetcode.com/problems/generate-parentheses/

题意：

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:

"((()))", "(()())", "(())()", "()(())", "()()()"

解题思路：列举出所有合法的括号匹配，使用dfs。如果左括号的数量大于右括号的数量的话，就不能产生合法的括号匹配。

代码：

class Solution:
# @param an integer
# @return a list of string
# @draw a decision tree when n == 2, and you can understand it!
def helpler(self, l, r, item, res):
if r < l:
return
if l == 0 and r == 0:
res.append(item)
if l > 0:
self.helpler(l - 1, r, item + '(', res)
if r > 0:
self.helpler(l, r - 1, item + ')', res)

def generateParenthesis(self, n):
if n == 0:
return []
res = []
self.helpler(n, n, '', res)
return res