https://leetcode.com/problems/generate-parentheses/discuss/10110/Simple-Python-DFS-solution-with-explanation
1 两个stack，一个存左括号，一个存右括号
先产生一对()；再产生0-pair inside，n-1-pair outside；1-pair inside，n-2-pair outside；2-pair inside，n-3-pair outside
2 dp方法：https://leetcode.com/problems/generate-parentheses/discuss/10369/Clean-Python-DP-Solution
当n=3的时候，dp[0] = [""], dp[1]=["()"], dp[2]=["()()","(())"]，dp[3]=["()()()","()(())","(())()","(()())","((()))"]
