每日一题 --- 606. 根据二叉树创建字符串[力扣][Go]

func tree2str(root *TreeNode) string {
  var ans strings.Builder
  var dfs func(*TreeNode)
  dfs = func(node *TreeNode) {
    if node == nil {
      return
    }
    ans.WriteString(strconv.Itoa(node.Val))
    if node.Right != nil {
      ans.WriteString("(")
      dfs(node.Left)
      ans.WriteString(")")
      ans.WriteString("(")
      dfs(node.Right)
      ans.WriteString(")")
    } else if node.Left != nil {
      ans.WriteString("(")
      dfs(node.Left)
      ans.WriteString(")")
    }
  }
  dfs(root)
  return ans.String()
}