def countSteps(n):
    dp = [0] * (n + 1)
    dp[1] = 1
    dp[2] = 2
    dp[3] = 4
    for i in range(4, n + 1):
        dp[i] = dp[i-1] + dp[i-2] + dp[i-3]
    return dp[n]

# 输入
n = int(input())

# 计算并输出结果
result = countSteps(n)
print(result)

2 个 RE ，其他都 AC ...

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