Find the second most frequent tag

Q: Find the second most frequent tag

This question evaluates a candidate's ability to perform frequency analysis and stable tie-breaking on a linear sequence, exercising skills in hashing, counting, and handling edge cases such as equal frequencies.

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Question

You are given a list of strings in the following repeating order:

[id1, name1, tag1, id2, name2, tag2, ...]

So every 3 consecutive elements describe one record: (id, name, tag).

Return the tag that is the second most frequent among all records’ tags.

Tie/edge-case rules:

If there is no “second most frequent” tag because all tags have the same count (i.e., there is only one distinct frequency), return "notag" .
If there are multiple tags tied for second most frequent, return the one whose first appearance in the input list occurs earliest.

Constraints/expectations:

Run time should be O(n).
You may assume the input length is a multiple of 3.

Example:

Input: ["1","A","x","2","B","y","3","C","y","4","D","x","5","E","z"] Counts: x=2, y=2, z=1 → second most frequent is z → return "z".

Write a function to compute the required tag.

Find the second most frequent tag

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