A smart traffic system uses three signal states: 0 (red — stop), 1 (yellow — caution), and 2 (green — go). A log of n signal readings has arrived in a random order. The system needs to sort the log in-place so all red signals come first, then yellow, then green — using only a single pass and O(1) extra memory.

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KaTeX can only parse string typed expression space-separated integers — the sorted sequence.