Hash Map / Set

Hash maps and sets trade memory for O(1) average lookups, insertions, and membership tests. Reach for one whenever you catch yourself asking 'have I seen this before?' or 'how many times?' — they turn nested-loop searches into a single pass.

22 LeetCode problems solved with the Hash Map / Set pattern. Practice them with spaced repetition so the pattern sticks.

• 4Sum IIMEDIUM · O(n^2)
• Contains DuplicateEASY · O(n)
• Contains Duplicate IIEASY · O(n)
• Count Good MealsMEDIUM · O(n * log maxSum)
• Determine if Two Strings Are CloseMEDIUM · O(n + k log k)
• Equal Row and Column PairsMEDIUM · O(n^2)
• Find the Difference of Two ArraysEASY · O(n + m)
• First Missing PositiveHARD · O(n)
• Group AnagramsMEDIUM · O(n*k log k)
• Intersection of Two ArraysEASY · O(n + m)
• Isomorphic StringsEASY · O(n)
• Longest Consecutive SequenceMEDIUM · O(n)
• Longest PalindromeEASY · O(n)
• Majority Element IIMEDIUM · O(n)
• Max Number of K-Sum PairsMEDIUM · O(n)
• Max Points on a LineHARD · O(n^2)
• Ransom NoteEASY · O(m + n)
• Two SumEASY · O(n)
• Unique Number of OccurrencesEASY · O(n)
• Valid AnagramEASY · O(n)
• Valid SudokuMEDIUM · O(1)
• Word PatternEASY · O(n + m)