Must-do problems: Maps

180 min · Intermediate

Every problem here is solved with a map: a frequency map, a grouping map, a lookup map, or a map that remembers what was seen so far. Read the input with Scanner, decide first what the key and the value are, pick the map that gives the output order the problem asks for, and run every test before opening a hint.

What you will learn

Count, total and group data with a map in one pass
Use a map to remember what was seen so far (first repeat, two-sum, prefix sums)
Choose HashMap, LinkedHashMap or TreeMap for the output order a problem needs
Build keys for grouping, such as sorted letters for anagrams
Solve an LRU cache with an access-order LinkedHashMap

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