HTML Elements Single-Tag Correction

Given a string str containing plain text and HTML-like tags, determine whether its tags are properly nested. The only tag names used are b, i, em, div, and p.
A string is properly nested when every closing tag matches the most recently opened tag that has not yet been closed.
If str is properly nested, return the string true. Otherwise, exactly one tag name can be changed to a different allowed tag name to make the string properly nested. The tag remains opening or closing after the change. Return the original name of the earliest tag token whose name can be changed to make the whole string properly nested.

Function
htmlElements(str: String) → String

Examples
Example 1
str = "<div><b><p>hello world</p></b></div>"
return = "true"
Each closing tag matches the most recently opened tag, so the string is already properly nested.
Example 2
str = "<div><i>hello</i>world</b>"
return = "div"
Changing the first <div> tag to <b> makes the string properly nested, so return its original name, div.
Example 3
str = "<div><div><b></b></div></p>"
return = "div"
Changing the first <div> tag to <p> makes all tags match. Although changing the final closing tag also works, the first tag occurs earlier.
Example 4
str = "<div>abc</div><p><em><i>test test test</b></em></p>"
return = "i"
Changing <i> to <b> makes the second top-level element properly nested.

Constraints
Every tag name is one of b, i, em, div, or p.
If str is not already properly nested, changing exactly one tag name can make it properly nested.

Parse the tags with a regex or manual scan, keeping opening and closing as separate token types. Run a standard stack. On the first mismatch, a closing tag meets a different opener on top of the stack. That's your conflict, but the culprit isn't always the closing tag. Example 3 proves it: renaming the earlier opening div works, and the earliest valid token wins. So at the mismatch you have two candidates, the stack top's opener or the closer, and the answer is the opener's original name since it appears earlier. Pitfall: the opener you blame may not be the top. Verify by simulating the rename, or test each candidate token in order against a full check. Since the constraints guarantee one rename fixes it, brute force over tokens and 4 alternative names is fine for small inputs. If you freeze mid-OA, StealthCoder is the hedge, but the mismatch logic is only about ten lines.

Drill it cold or hedge it with StealthCoder. Either way, don't walk into the OA hoping you remember the trick.