摘要：
We have two integer sequences A and B of the same non-zero length. We are allowed to swap elements A[i] and B[i]. Note that both elements are in the s 阅读全文
摘要：
We have two types of tiles: a 2x1 domino shape, and an "L" tromino shape. These shapes may be rotated. Given N, how many ways are there to tile a 2 x 阅读全文