Generate all Parentheses II

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses of length 2*n.

For example, given n = 3, a solution set is:

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

Make sure the returned list of strings are sorted.

NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a doubt? Checkout Sample Codes for more details.