Project Euler 第28题

Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:

21  22  23  24 25
20  7    8    9   10
19  6   1    2   11
18  5    4    3   12
17 16  15  14   13

It can be verified that the sum of both diagonals is 101.

What is the sum of both diagonals in a 1001 by 1001 spiral formed in the same way?

直接总结下规律即可。

圈数为奇数，步长为2。每一圈有4个数，每个数构成等差数列，差为n-1。