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Practice Lab

Live Coding Drills

Write code, run tests, and learn the core patterns with instant feedback.

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Pattern Coach
Follow the flow
  • 1
    Read the pattern card
    What it is, when to use it, traps.
  • 2
    Answer the thinking prompts
    Make the invariant explicit.
  • 3
    Write a short plan
    3 steps max, then code.
  • 4
    Run tests + fix
    Learn from mistakes fast.
Tip: Say the invariant out loud before you code.
Two pointers
Two Sum (Sorted)
Given a sorted array and a target, return true if a pair sums to the target.
+120 XP
Function signature
function hasPairSum(nums, target) {}
hasPairSum.jsjavascript
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Pattern card
Two pointers moving inward while keeping an invariant.
Use it when
Sorted arrays or monotonic pointer moves.
Common traps
  • • Off-by-one bounds
  • • Moving the wrong pointer
Hints
Try a hint, then re-run tests. Use the next hint only if you still fail.
  • • Sorted array means pointer moves change the sum directionally.
  • • Shrink the window when sum is too big.
Common mistakes
  • • Moving both pointers at once.
  • • Forgetting to stop when left >= right.
Critical thinking
Questions to answer before you code
  • • What invariant keeps the window valid?
  • • Which pointer move reduces the sum?
Plan
  • • Set left=0, right=n-1.
  • • Move pointers based on sum vs target.
  • • Stop when left >= right.
Edge cases
  • • Empty array
  • • Two elements only
  • • Target not possible
Target: O(n) time, O(1) space
If you are stuck, rewrite the plan in your own words before coding.
Test results
Run tests to see results.
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