青蛙过河（互换），[题目描述] 一共有2n+1块石头，左右各有两队、各N只青蛙，当当中一块石头空白，怎样将左面的青蛙与右面的青蛙互换呢? 如下图所示（其中X表示空格）

-Frogger (interchangeable), [Title Descr iption] and a total 2n+1 stones, each about two teams, each N only frog in Dangdang stone blank, how the frog on the left and the right frog interchangeable? Following figure shown (where X represents a space)