Practice · 3 of 4

Clinic: The Left-Wall Only

CHALLENGE

Difficulty: advanced+25 XP

**Bài toán.** n ≤ 200000 bar heights; compute trapped rainwater total. **The submission below accumulates water using only the running max from the left — Wrong Answer.** Construct the counterexample (hint: a tall wall on the right changes everything), then fix it.

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Back to lesson: Practice: Clinic Set 2 — Subtle Failures