如何为字符串补全首尾空格？单侧已有则补另一侧，无则双侧补

Hey folks! If you need to format content using Markdown, here's a straightforward, easy-to-follow guide with all the key rules you'll need:

• Headings: Use number signs (#) at the start of a line. The number of #s dictates the heading level—more signs mean smaller headings:

  • # Level 1 Heading (biggest)
  • ## Level 2 Heading
  • ### Level 3 Heading
  • ... all the way up to ###### Level 6 Heading (smallest)

• List Items: Kick off each list entry with a dash (-). Want nested lists? Just indent the inner items:

  • Top-level list item
    • Nested list item
    • Another nested item

• Emphasized Text: Wrap text you want to highlight with single asterisks (*) for italicized emphasis:

  • Example: *This text is italicized* shows up as This text is italicized

• Code & Commands: For inline code or commands, wrap them in backticks (`). For multi-line code blocks, use three backticks (```) on lines before and after your code:

  • Inline example: git commit -m "Fix bug"
  • Multi-line example:

    def calculate_sum(a, b):
        return a + b
    

• Quoted Text: Prepend any line you want to quote with a greater-than sign (>):

  • Example:

    This is a quoted paragraph.
    It can span multiple lines seamlessly.

• Links: Structure links like this: wrap the text you want people to click in square brackets ([]), then add the target URL right after in parentheses (()):

  • Example: [Markdown Guide](https://www.markdownguide.org) becomes Markdown Guide

• Images: Similar to links, but start with an exclamation mark (!). Put descriptive alt text (for accessibility) in square brackets, followed by the image URL in parentheses:

  • Example: ![Sunset over mountains](https://example.com/sunset.jpg)

内容的提问来源于stack exchange，提问作者EyTa