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题目描述

给出 $n(n\le20)$，输出杨辉三角的前 $n$ 行。

如果你不知道什么是杨辉三角，可以观察样例找找规律。

6

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1