PUMaC 2025 · 组合（B 组） · 第 3 题
PUMaC 2025 — Combinatorics (Division B) — Problem 3
题目详情
- A string of 0s, 1s, and 2s is called nice if it doesn’t contain “01” or “12”, doesn’t begin with a “2”, and doesn’t end with a “0”. For example, there are 5 nice strings of length 3, namely “002”, “021”, “022”, “102”, and “111”. Find the number of nice strings of length 7.
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