Generate Parentheses

def generateParenthesis(n):
    stack = []
    res = []
    
    def backtrack(openN, closedN):
        if openN == closedN == n:
            res.append("".join(stack))
            return
            
        if openN < n:
            stack.append("(")
            backtrack(openN + 1, closedN)
            stack.pop()
            
        if closedN < openN:
            stack.append(")")
            backtrack(openN, closedN + 1)
            stack.pop()
            
    backtrack(0, 0)
    return res