最后输出如下a1b1c1d1a1b1c1d2a1b1c1d3a1b1c1d4a1b1c2d1a1b1c2d2a1b1c2d3a1b2c2d4a1b1c3d1a1b1c3d2a1b1c3d3a1b1c3d4如此类推