每日一题---22. 括号生成[力扣][Go]

func generateParenthesis(n int) []string {
  if n == 0{
    return nil
  }
  var sum []string
  generateParenthesisStep(&sum,"",n,0,0)
  return sum
}
func generateParenthesisStep(sum *[]string,str string,n,r,l int) {
  if r > n || l > n || r > l {
    return
  }
  if r == n && l == n {
    *sum = append(*sum, str)
  }
  generateParenthesisStep(sum,str +"(",n,r,l+1)
  generateParenthesisStep(sum,str +")",n,r+1,l)
  return
}