How should I practice Parse Nested HTML into a Tree?

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Parse Nested HTML into a Tree

Apple● MediumFULLTIMEONSITE INTERVIEW

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Problem statement

Parse a well-formed markup string into its nested element tree and return the element names in depth-first preorder.

The grammar contains paired lowercase tags such as <section>...</section> and arbitrary text leaves. Attributes, comments, entities, void elements, and self-closing tags are outside scope. The input has exactly one root element.

Function

preorderElementNames(html: String) → String[]

Examples

Example 1

html = "<div>Hello<span>world</span></div>"return = ["div","span"]

Preorder visits the root div before its nested span.

Example 2

html = "<a><b></b><c><d></d></c></a>"return = ["a","b","c","d"]

Depth-first preorder follows a, b, c, then d.

Constraints

7 <= html.length <= 200000.
Tag names contain one to twenty lowercase ASCII letters.
The markup is well formed and has one root element.