Generate Parenthesis@leetcode

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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:

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

一个小破题，dp还有recursion，关键是思路要清楚还有recursion要很熟练。
先排左边括号，再看中间所有的可能，就是top down，再botton up。先编织最外面一层，再进入到最里面，从每个基本情况排列起来。可以和permutation, subsets对比，都是recursion和dp的题目。