{"id":6884,"date":"2025-06-17T19:04:43","date_gmt":"2025-06-17T13:34:43","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=6884"},"modified":"2025-06-17T19:04:44","modified_gmt":"2025-06-17T13:34:44","slug":"detect-linked-list-cycle-using-hash-table","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/detect-linked-list-cycle-using-hash-table\/","title":{"rendered":"Detect Linked List Cycle Using Hash Table"},"content":{"rendered":"\n<!-- PrismJS for Syntax Highlighting -->\n<link href=\"https:\/\/cdn.jsdelivr.net\/npm\/prismjs@1.29.0\/themes\/prism-tomorrow.min.css\" rel=\"stylesheet\">\n<script src=\"https:\/\/cdn.jsdelivr.net\/npm\/prismjs@1.29.0\/prism.min.js\"><\/script>\n<script src=\"https:\/\/cdn.jsdelivr.net\/npm\/prismjs@1.29.0\/plugins\/autoloader\/prism-autoloader.min.js\"><\/script>\n\n<style>\n    .wp_blog_main-heading {\n      text-align: center;\n      font-size: 2.4rem;\n      color: #E58C32;\n      margin-top: 2.5rem;\n      font-weight: bold;\n    }\n    \n    .wp_blog_explanation,\n    .wp_blog_code-tabs-container {\n      max-width: 940px;\n      margin: 2rem auto;\n      padding: 2rem;\n      border-radius: 12px;\n      background-color: #f9fafb;\n    }\n    \n    .wp_blog_explanation h2 {\n      font-size: 1.4rem;\n      color: #E58C32;\n      margin-bottom: 0.75rem;\n    }\n    \n    .wp_blog_explanation p,\n    .wp_blog_explanation li {\n      font-size: 1.05rem;\n      line-height: 1.7;\n      margin: 0.5rem 0;\n      color: #1f2937;\n    }\n    \n    .wp_blog_explanation code {\n      padding: 3px 6px;\n      border-radius: 4px;\n      background: #e5e7eb;\n      font-family: 'Courier New', monospace;\n    }\n    \n    .wp_blog_code-tabs-header {\n      display: flex;\n      flex-wrap: wrap;\n      gap: 0.5rem;\n      margin-bottom: 1rem;\n    }\n    \n    .wp_blog_code-tab-button {\n      padding: 0.6rem 1.2rem;\n      border: 1px solid #E58C32;\n      color: #E58C32;\n      border-radius: 50px;\n      font-weight: 600;\n      cursor: pointer;\n      background-color: white;\n      transition: background 0.3s ease, color 0.3s ease;\n    }\n    \n    .wp_blog_code-tab-button.active {\n      background: #E58C32;\n      color: white;\n    }\n    \n    .wp_blog_code-tab-content {\n      display: none;\n      background: #111827;\n      border-radius: 12px;\n    }\n    \n    .wp_blog_code-tab-content.active {\n      display: block;\n    }\n    \n    .wp_blog_code-tab-content pre {\n      margin: 0;\n      padding: 1.5rem;\n      font-size: 1rem;\n      overflow-x: auto;\n      color: #f3f4f6;\n      background: #111827;\n      border-radius: 12px;\n    }\n    <\/style>\n\n<div class=\"wp_blog_explanation\">\n  <h2>Problem Statement:<\/h2>\n  <p>\n    Given <code>head<\/code>, the head of a linked list, determine if the linked list has a cycle in it.\n    Return <code>true<\/code> if there is a cycle; otherwise, return <code>false<\/code>.\n  <\/p>\n\n  <h2>Examples:<\/h2>\n  <ul>\n    <li><strong>Input:<\/strong> head = [3,2,0,-4], pos = 1<br><strong>Output:<\/strong> true<\/li>\n    <li><strong>Input:<\/strong> head = [1,2], pos = 0<br><strong>Output:<\/strong> true<\/li>\n    <li><strong>Input:<\/strong> head = [1], pos = -1<br><strong>Output:<\/strong> false<\/li>\n  <\/ul>\n\n  <h2>Constraints:<\/h2>\n  <ul>\n    <li>The number of nodes is in the range [0, 10\u2074].<\/li>\n    <li>-10\u2075 \u2264 Node.val \u2264 10\u2075<\/li>\n    <li><code>pos<\/code> is -1 or a valid index in the linked list.<\/li>\n  <\/ul>\n\n  <h2>Approach:<\/h2>\n  <ul>\n    <li>Use a <code>Set<\/code> to track visited nodes.<\/li>\n    <li>While traversing, if we encounter a node already in the set, we\u2019ve found a cycle.<\/li>\n    <li>If we reach <code>null<\/code>, there&#8217;s no cycle.<\/li>\n  <\/ul>\n\n  <h2>Time and Space Complexity:<\/h2>\n  <ul>\n    <li><strong>Time Complexity:<\/strong> O(n), where n is the number of nodes in the list.<\/li>\n    <li><strong>Space Complexity:<\/strong> O(n), in the worst case we store all nodes in a set.<\/li>\n  <\/ul>\n<\/div>\n\n<div class=\"wp_blog_code-tabs-container\">\n  <div class=\"wp_blog_code-tabs-header\">\n    <button class=\"wp_blog_code-tab-button active\" data-lang=\"js\">JavaScript<\/button>\n    <button class=\"wp_blog_code-tab-button\" data-lang=\"py\">Python<\/button>\n    <button class=\"wp_blog_code-tab-button\" data-lang=\"java\">Java<\/button>\n    <button class=\"wp_blog_code-tab-button\" data-lang=\"cpp\">C++<\/button>\n    <button class=\"wp_blog_code-tab-button\" data-lang=\"c\">C<\/button>\n    <button class=\"wp_blog_code-tab-button\" data-lang=\"cs\">C#<\/button>\n  <\/div>\n\n  <!-- JavaScript -->\n  <div class=\"wp_blog_code-tab-content active\" data-lang=\"js\">\n    <pre><code class=\"language-javascript\">\nvar hasCycle = function(head) {\n    let seenNodes = new Set();\n    let curr = head;\n    while (curr !== null) {\n        if (seenNodes.has(curr)) {\n            return true;\n        }\n        seenNodes.add(curr);\n        curr = curr.next;\n    }\n    return false;\n};\n    <\/code><\/pre>\n  <\/div>\n\n  <!-- Python -->\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"py\">\n    <pre><code class=\"language-python\">\nclass Solution(object):\n    def hasCycle(self, head):\n        seen = set()\n        while head:\n            if head in seen:\n                return True\n            seen.add(head)\n            head = head.next\n        return False\n    <\/code><\/pre>\n  <\/div>\n\n  <!-- Java -->\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"java\">\n    <pre><code class=\"language-java\">\npublic class Solution {\n    public boolean hasCycle(ListNode head) {\n        Set&lt;ListNode&gt; seen = new HashSet&lt;&gt;();\n        while (head != null) {\n            if (seen.contains(head)) {\n                return true;\n            }\n            seen.add(head);\n            head = head.next;\n        }\n        return false;\n    }\n}\n    <\/code><\/pre>\n  <\/div>\n\n  <!-- C++ -->\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"cpp\">\n    <pre><code class=\"language-cpp\">\nclass Solution {\npublic:\n    bool hasCycle(ListNode *head) {\n        unordered_set&lt;ListNode*&gt; seen;\n        while (head != nullptr) {\n            if (seen.count(head)) return true;\n            seen.insert(head);\n            head = head->next;\n        }\n        return false;\n    }\n};\n    <\/code><\/pre>\n  <\/div>\n\n  <!-- C -->\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"c\">\n    <pre><code class=\"language-c\">\nbool hasCycle(struct ListNode *head) {\n    struct ListNode *slow = head, *fast = head;\n    while (fast && fast->next) {\n        slow = slow->next;\n        fast = fast->next->next;\n        if (slow == fast) return true;\n    }\n    return false;\n}\n    <\/code><\/pre>\n  <\/div>\n\n  <!-- C# -->\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"cs\">\n    <pre><code class=\"language-csharp\">\npublic class Solution {\n    public bool HasCycle(ListNode head) {\n        HashSet&lt;ListNode&gt; seen = new HashSet&lt;ListNode&gt;();\n        while (head != null) {\n            if (seen.Contains(head)) {\n                return true;\n            }\n            seen.Add(head);\n            head = head.next;\n        }\n        return false;\n    }\n}\n    <\/code><\/pre>\n  <\/div>\n<\/div>\n\n<script>\n    document.addEventListener('DOMContentLoaded', () => {\n    const buttons = document.querySelectorAll('.wp_blog_code-tab-button');\n    const contents = document.querySelectorAll('.wp_blog_code-tab-content');\n    \n    buttons.forEach(button => {\n    button.addEventListener('click', () => {\n    const lang = button.getAttribute('data-lang');\n    buttons.forEach(btn => btn.classList.remove('active'));\n    contents.forEach(content => content.classList.remove('active'));\n    button.classList.add('active');\n    document.querySelector(`.wp_blog_code-tab-content[data-lang=\"${lang}\"]`).classList.add('active');\n    });\n    });\n    });\n    <\/script>\n\n","protected":false},"excerpt":{"rendered":"<p>Problem Statement: Given head, the head of a linked list, determine if the linked list has a cycle in it. Return true if there is a cycle; otherwise, return false. Examples: Input: head = [3,2,0,-4], pos = 1Output: true Input: head = [1,2], pos = 0Output: true Input: head = [1], pos = -1Output: false<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[260,176,811,810,174,172,173],"tags":[],"class_list":["post-6884","post","type-post","status-publish","format-standard","category-c-c-plus-plus","category-csharp","category-data-structures-and-algorithms","category-dsa","category-java","category-javascript","category-python"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/6884","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/comments?post=6884"}],"version-history":[{"count":1,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/6884\/revisions"}],"predecessor-version":[{"id":6885,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/6884\/revisions\/6885"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=6884"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=6884"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=6884"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}