{"id":8862,"date":"2021-11-28T13:23:00","date_gmt":"2021-11-28T21:23:00","guid":{"rendered":"https:\/\/zxi.mytechroad.com\/blog\/?p=8862"},"modified":"2021-11-28T13:28:45","modified_gmt":"2021-11-28T21:28:45","slug":"leetcode-142-linked-list-cycle-ii","status":"publish","type":"post","link":"https:\/\/zxi.mytechroad.com\/blog\/list\/leetcode-142-linked-list-cycle-ii\/","title":{"rendered":"\u82b1\u82b1\u9171 LeetCode 142. Linked List Cycle II"},"content":{"rendered":"\n

Given the head<\/code> of a linked list, return the node where the cycle begins. If there is no cycle, return <\/em>null<\/code>.<\/p>\n\n\n\n

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next<\/code> pointer. Internally, pos<\/code> is used to denote the index of the node that tail’s next<\/code> pointer is connected to (0-indexed<\/strong>). It is -1<\/code> if there is no cycle. Note that<\/strong> pos<\/code> is not passed as a parameter<\/strong>.<\/p>\n\n\n\n

Do not modify<\/strong> the linked list.<\/p>\n\n\n\n

Example 1:<\/strong><\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n
Input:<\/strong> head = [3,2,0,-4], pos = 1\nOutput:<\/strong> tail connects to node index 1\nExplanation:<\/strong> There is a cycle in the linked list, where tail connects to the second node.\n<\/pre>\n\n\n\n
Example 2:<\/strong><\/p>\n\n\n\n
\"\"\/<\/figure>\n\n\n\n
Input:<\/strong> head = [1,2], pos = 0\nOutput:<\/strong> tail connects to node index 0\nExplanation:<\/strong> There is a cycle in the linked list, where tail connects to the first node.\n<\/pre>\n\n\n\n
Example 3:<\/strong><\/p>\n\n\n\n
\"\"\/<\/figure>\n\n\n\n
Input:<\/strong> head = [1], pos = -1\nOutput:<\/strong> no cycle\nExplanation:<\/strong> There is no cycle in the linked list.\n<\/pre>\n\n\n\n
Constraints:<\/strong><\/p>\n\n\n\n
  • The number of the nodes in the list is in the range [0, 104<\/sup>]<\/code>.<\/li>
  • -105<\/sup> <= Node.val <= 105<\/sup><\/code><\/li>
  • pos<\/code> is -1<\/code> or a valid index<\/strong> in the linked-list.<\/li><\/ul>\n\n\n\n
    Follow up:<\/strong> Can you solve it using O(1)<\/code> (i.e. constant) memory?<\/p>\n\n\n\n
    <\/p>\n\n\n\n
    Solution 1: Hashtset<\/strong><\/h2>\n\n\n\n
    Time complexity: O(n)
    Space complexity: O(n)<\/p>\n\n\n\n
    \n
    C++<\/h2>\n
    \n\n
    \n\/\/ Author: Huahua\nclass Solution {\npublic:\n  ListNode *detectCycle(ListNode *head) {\n    unordered_set s;\n    for (ListNode* cur = head; cur; cur = cur->next)\n      if (!s.insert(cur).second) return cur;\n    return nullptr;\n  }\n};\n<\/pre>\n<\/div><\/div>\n\n\n\n
    Solution: Fast slow pointers<\/strong><\/h2>\n\n\n\n
    Time complexity: O(n)
    Space complexity: O(1)<\/p>\n\n\n\n
    \n
    C++<\/h2>\n
    \n\n
    \n\/\/ Author: Huahua\nclass Solution {\npublic:\n  ListNode *detectCycle(ListNode *head) {\n    unordered_set s;\n    for (ListNode* cur = head; cur; cur = cur->next)\n      if (!s.insert(cur).second) return cur;\n    return nullptr;\n  }\n};\n<\/pre>\n<\/div><\/div>\n","protected":false},"excerpt":{"rendered":"
    Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null. There is a cycle in a linked list…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[50],"tags":[123,82,83,177],"class_list":["post-8862","post","type-post","status-publish","format-standard","hentry","category-list","tag-cycle","tag-hashtable","tag-list","tag-medium","entry","simple"],"_links":{"self":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts\/8862","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/comments?post=8862"}],"version-history":[{"count":2,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts\/8862\/revisions"}],"predecessor-version":[{"id":8865,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts\/8862\/revisions\/8865"}],"wp:attachment":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/media?parent=8862"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/categories?post=8862"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/tags?post=8862"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}