排列组合视频讲解：https://www.bilibili.com/video/BV1YCQDYTEmM?spm_id_from=333.788.videopod.sections&vd_source=ed609f9463861996afb742d2bd8b5a46
排列组合专项基础：https://www.bilibili.com/video/BV1MF411w7rR?spm_id_from=333.788.videopod.sections&vd_source=ed609f9463861996afb742d2bd8b5a46
排列组合专项：https://www.bilibili.com/video/BV1rB4y137UU?spm_id_from=333.788.videopod.sections&vd_source=ed609f9463861996afb742d2bd8b5a46