题解 | 小红的三带一

def solve(testcase):
    n = II()
    A = LII()

    dp = [0 for _ in range(n + 1)]

    def f(idx):
        c = Counter(A[idx - 3: idx + 1])
        if len(c) == 2 and sorted(list(c.values())) == [1, 3]:
            return True
        else:
            return False


    for i in range(3, n):
        dp[i + 1] = dp[i]
        if f(i):
            dp[i + 1] = fmax(dp[i + 1], dp[i - 3] + 1)
    
    print(dp[n])

for testcase in range(1):
    solve(testcase)