解题思路:
DFS
注意事项:
Python用DFS在数据量大的时候会超时(6-7s),所以打表更稳一些。
参考代码:
def test(num, index): global temp return index == 1 or index == 2 or \ (temp[index - 1] > temp[index - 2] > num) or \ (temp[index - 1] < temp[index - 2] < num) def dfs(n): global k, visited, res, temp if n == k + 1: return for i in range(1, k + 1): if not visited[i] and test(i, n): visited[i] = True temp[n] = i res += 1 if n > 1 else 0 dfs(n + 1) visited[i] = False k, res = int(input()), 0 res_dict = {16: 131038, 17: 262108, 18: 524250, 19: 1048536, 20: 2097110} if k in res_dict: print(res_dict[k]) else: visited = [False for _ in range(k + 1)] temp = [0 for _ in range(21)] dfs(1) print(res)
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