回溯法介绍

 https://www.jianshu.com/p/3f8ada396b9f

https://blog.csdn.net/Herishwater/article/details/98989512?depth_1-utm_source=distribute.pc_relevant.none-task-blog-BlogCommendFromBaidu-1&utm_source=distribute.pc_relevant.none-task-blog-BlogCommendFromBaidu-1

时间复杂度2^n-1

叶子节点数为2^(n-1)