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: "((()))", "(()())", "(())()", "()(())", "()()()".

Solution: If we have k < n left brackets, we can add a left bracket.
If the number of right brackets < the number of left brackets, we can add a right bracket.

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