[数据结构与算法]链表中为什么要设置虚拟头结点dummy？

传送门：
https://blog.csdn.net/weixin_43996641/article/details/103384555?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522163647319716780274131937%2522%252C%2522scm%2522%253A%252220140713.130102334.pc%255Fall.%2522%257D&request_id=163647319716780274131937&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2_allfirst_rank_ecpm_v1~rank_v31_ecpm-2-103384555.pc_search_result_cache&utm_term=%E8%99%9A%E6%8B%9F%E5%A4%B4%E7%BB%93%E7%82%B9&spm=1018.2226.3001.4187