这是计算nCr的更好方法

def nCr(n, r):
    C = [[0 for j in range(r+1)] for i in range(n+1)]
    for i in range(n+1):
        for j in range(min(i, r)+1):
            if j == 0 or j == i:
                C[i][j] = 1
            else:
                C[i][j] = C[i-1][j-1] + C[i-1][j]
    return C[n][r]