Storm Drain Skyline Audit
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Storm Drain Skyline Audit
A city platform stores a skyline scan after a storm. Each value in walls represents the height of one wall segment from left to right. Water may rest above a segment only if there is a boundary somewhere to its left and a boundary somewhere to its right. The amount of water held above a segment depends on the lower of the best boundary on both sides. Segments at the edges may hold no water unless the surrounding boundaries make it possible. Input Format: You are given an integer array walls. Output Format: Return the total number of water units trapped after the storm settles.
Examples
walls = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]
6
Explanation: The trapped water above each index is [0, 0, 1, 0, 1, 2, 1, 0, 0, 1, 0, 0]. Their sum is 6, so the output is 6.
Approach hint
Look for the state that actually matters after each operation.
Common mistake
Skipping assumptions, edge cases, or trade-offs can make an otherwise good answer feel incomplete.