Given a string containing only digits, restore it by returning all possible valid IP address combinations. For example: Given "25525511135", return ["255.255.11.135", "255.255.111.35"]. (Order do
Given a linked list, return the node where the cycle begins. If there is no cycle, return null. Note: Do not modify the linked list. 当fast和slow相遇时,slow肯定没有遍历完链表,而fast已经在环内循环了n圈。假设slow走了s步,则fast走了2s步