https://leetcode.com/problems/linked-list-cycle/description/ https://leetcode.com/problems/linked-list-cycle-ii/description/ https://leetcode-cn.com/problems/reverse-linked-list/description https://leetcode-cn.com/problems/reverse-linked-list-ii/description/ https://leetcode-cn.com/problems/swap-nodes-in-pairs/description/ https://leetcode-cn.com/problems/add-two-numbers/description/ https://leetcode-cn.com/problems/remove-duplicates-from-sorted-list/description/ https://leetcode-cn.com/problems/remove-duplicates-from-sorted-list-ii/description/ https://leetcode-cn.com/problems/merge-two-sorted-lists/description/