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,当中一块石头空白,如何将左面的青蛙和右面的青蛙互换呢? 如下图所示（其中X表示空格） -Frogger (exchange), [Title Descr iption] A total of 2n+ 1 stones, each about two teams, the N frogs, of which a stone blank, how to the left of the frog and the