python P3904 三只小猪

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题面
题面所求即为第二类斯特林数 

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n, m = map(int, input().split(' '))
a = [[0] * 55 for _ in range(55)]
if(m > n):
    print(0)
else:
    for i in range(1, n + 1):
        a[i][i] = a[i][1] = 1

    for j in range(2, m + 1):
        for i in range(j + 1, n + 1):
            a[i][j] = a[i - 1][j - 1] + a[i - 1][j] * j

    print(a[n][m])