def count_ways(n):
# 初始化 dp 数组
dp = [0] * (n + 1)
# 填充初始值
dp[0], dp[1], dp[2] = 1, 1, 2
# 计算 dp[n]
for i in range(3, n + 1):
dp[i] = dp[i-1] + dp[i-2] + dp[i-3]
return dp[n]

测试

n=int(input())
print(count_ways(n))