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Two Pointers Ladder

Two-pointers practice should teach you to recognize monotone windows and sorted scans, and just as importantly, to reject the pattern when the condition is not monotone enough.

Who This Is For

Use this ladder if:

  • you can copy a sliding-window loop, but not justify it
  • you are unsure when negative values break the approach
  • you want better instinct for window invariants

Warm-Up

  • fixed-size windows
  • count pairs in a sorted array
  • longest subarray under a monotone constraint

Target skill:

  • understand what the current window represents

Core

  • variable-size windows with counts or sums
  • distinct-element windows
  • shortest valid window
  • sorted two-end scans

Target skill:

  • know exactly when to expand, shrink, and update the answer

Starter Drill

For each new window problem, write down:

  • what [l, r] or [l, r) means
  • what summary the window stores
  • what makes the window invalid
  • when the answer is updated

Stretch

  • decide whether the condition is monotone enough for two pointers
  • compare two pointers against prefix sums plus binary search
  • explain why each pointer moves at most n times

Target skill:

  • prove linearity and detect invalid uses of the pattern

Retrieval Layer

Repo Anchors

Exit Criteria

You are ready to move on when:

  • you can state the maintained invariant before coding
  • you know whether the method depends on nonnegative values or sorted order
  • you can explain why no pointer ever needs to move backward

External Practice