22. Generate Parentheses

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:

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

AC代码：

class Solution(object):
    def generateParenthesis(self, n):
        if n < 1: return ['']
        def backtracing(left, right, current, ret_list):
            if left > 0:
                backtracing(left - 1, right, current + '(', ret_list)
            if right > 0:
                if left == right: return
                backtracing(left, right - 1, current + ')', ret_list)
            else:
                ret_list.append(current)
        ret_list = []
        backtracing(n, n, '', ret_list)
        return ret_list

用递归，找准递归出口即可。注意left==right的条件。