Must-do problems: Sets

150 min · Intermediate

Every problem here is about duplicates, common values, repeats or unique counts, and a set gives each one a short and fast solution. Read the input with Scanner, solve it with lists and sets only, and compare with the hints and solution when you are stuck.

What you will learn

Drop duplicates and count different values with a HashSet
Detect a repeat with the boolean returned by add
Use TreeSet and LinkedHashSet when the output order matters
Combine two collections with retainAll, removeAll and addAll
Solve missing-number and consecutive-run problems with contains

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