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CoursesDSA Foundations

Arrays & Hashing

Lessons
9
Exercises
57
Minutes
63

Start Array Traversal & Indexing

  1. 1

    Array Traversal & Indexing

    Read elements by index and walk an array from start to end with a loop.

    Complete codeDebugCode orderMultiple choiceTapTrace
    6 exercises
    6 min
  2. 2

    In-Place Modification

    Change an array without extra space: swap, overwrite, and reverse using indices.

    Complete codeDebugCode orderMultiple choiceTapTrace
    6 exercises
    7 min
  3. 3

    Array Basics CheckpointCheckpoint

    Prove you can index, traverse, and modify an array before moving on to two pointers.

    Complete codeDebugCode orderFill blankMultiple choiceTapTrace
    7 exercises
    8 min
  4. 4

    Hash Sets

    Use a set to answer 'have I seen this before?' in constant time.

    Complete codeDebugCode orderMultiple choiceTapTrace
    6 exercises
    7 min
  5. 5

    Frequency Counting

    Use a hash map to count how many times each item appears.

    Complete codeDebugCode orderMultiple choiceTapTrace
    6 exercises
    7 min
  6. 6

    Anagram Check

    Compare character frequencies to decide whether two strings are anagrams.

    Complete codeDebugCode orderFill blankMultiple choiceTrace
    6 exercises
    7 min
  7. 7

    Two Sum with a Hash Map

    Find a pair summing to a target in one pass using complement lookups.

    Complete codeDebugCode orderMultiple choiceTapTrace
    6 exercises
    7 min
  8. 8

    Arrays & Hashing ReviewReview

    Mixed recap: traversal, in-place swaps, hash sets, frequency counts, and complement lookups.

    Complete codeDebugFill blankMultiple choiceTapTrace
    6 exercises
    6 min
  9. 9

    Boss: Two SumBoss

    Solve Two Sum end to end: pick the pattern, build the one-pass hash map, and defend it on duplicates.

    Complete codeCode orderMultiple choiceTrace
    8 exercises
    8 min