Parentheses valid strings problem

https://leetcode.com/problems/generate-parentheses/?tab=Description

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:
`[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]`