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Practice SetLesson 201 min read

Tries — Practice Problems

November 05, 2026

// How to use this set

Work these in order: easy (Longest Common Prefix) → medium (Implement Trie, Add and Search Words, Replace Words, Map Sum Pairs, Longest Word in Dictionary, Search Suggestions System, Maximum XOR, Short Encoding of Words) → hard (Word Search II). Build the dict-of-dicts trie once and most of these are small variations on walking it. Cover the solution, solve it yourself, then type it out and run it. Every solution below was run and tested.

Ten problems that drill the trie pattern from Lesson 20. Build the dict-of-dicts trie once and most of these become small variations — a sentinel end-marker here, a DFS branch there. Each solution here was actually run and tested.

How to practise. For each problem, first decide what the nodes store (just children? a word-end flag? a stored value? a whole word?) — that decision is the problem. Then write the walk. If you stall for more than ~20 minutes, read the approach, understand it, and re-solve from memory the next day.

The 10 problems

  1. 1Longest Common Prefix (LeetCode 14)EasyFind the longest prefix shared by every string: build a trie and walk down while there is only one branch.arrow_forward
  2. 2Implement Trie (Prefix Tree) (LeetCode 208)MediumBuild insert, search and startsWith for a trie: one character walk over a dict of dicts with an end marker.arrow_forward
  3. 3Design Add and Search Words Data Structure (LeetCode 211)MediumStore words and search with '.' wildcards: a trie where a dot branches the DFS into every child.arrow_forward
  4. 4Replace Words (LeetCode 648)MediumReplace each word by its shortest dictionary root: put the roots in a trie and stop at the first end marker.arrow_forward
  5. 5Map Sum Pairs (LeetCode 677)MediumInsert key-value pairs and sum the values of all keys with a given prefix, using a trie plus a value map.arrow_forward
  6. 6Longest Word in Dictionary (LeetCode 720)MediumFind the longest word built one letter at a time from other words: sort, then grow a set of buildable words.arrow_forward
  7. 7Search Suggestions System (LeetCode 1268)MediumAfter each typed letter, suggest up to three products with that prefix: sort, then binary-search the prefix.arrow_forward
  8. 8Maximum XOR of Two Numbers in an Array (LeetCode 421)MediumFind the largest XOR of any pair: store numbers as bit paths in a trie and greedily take the opposite bit.arrow_forward
  9. 9Short Encoding of Words (LeetCode 820)MediumShortest reference string for a word list: insert words reversed into a trie and count only the leaves.arrow_forward
  10. 10Word Search II (LeetCode 212)HardFind every dictionary word on a letter grid: put words in a trie and let it prune one shared grid DFS.arrow_forward
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