Find the second most frequent tag

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: 1. 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"`. 2. 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.