P02 — Counting — every solution explained

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What is inside

1. Character frequency map, insertion-ordered
2. Most frequent character; ties -> earliest in the string
3. Does any character repeat?
4. Elements appearing exactly once, in first-seen order
5. How many DISTINCT values occur an odd number of times. (This is REV-001's reported "distinct after removing pairs".)
6. The single element appearing once when all others appear twice. XOR trick
7. First repeating element (earliest SECOND occurrence), else Integer.MIN_VALUE
8. Do two arrays contain the same multiset of values?
9. Highest frequency among all values
10. Words occurring more than once, in first-seen order, case-insensitive
11. Can the characters be rearranged into a palindrome?
12. Difference between the highest and lowest frequency present
13. Values occurring more than n/k times (k=3 -> "majority-ish")
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