{"id":6974,"date":"2025-06-18T21:02:24","date_gmt":"2025-06-18T15:32:24","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=6974"},"modified":"2025-06-18T21:02:25","modified_gmt":"2025-06-18T15:32:25","slug":"swap-nodes-in-pairs-recursive-approach","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/swap-nodes-in-pairs-recursive-approach\/","title":{"rendered":"Swap Nodes in Pairs ( Recursive approach )"},"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>Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list&#8217;s nodes (i.e., only nodes themselves may be changed.)<\/p>\n\n  <h2>Examples:<\/h2>\n  <p><strong>Input:<\/strong> head = [1,2,3,4]<br><strong>Output:<\/strong> [2,1,4,3]<\/p>\n  <p><strong>Input:<\/strong> head = []<br><strong>Output:<\/strong> []<\/p>\n  <p><strong>Input:<\/strong> head = [1]<br><strong>Output:<\/strong> [1]<\/p>\n\n  <h2>Constraints:<\/h2>\n  <ul>\n    <li>The number of nodes in the list is in the range [0, 100].<\/li>\n    <li>0 &le; Node.val &le; 100<\/li>\n  <\/ul>\n\n  <h2>Approach (Recursive):<\/h2>\n  <ul>\n    <li>Base case: if the list is empty or has only one node, return the head.<\/li>\n    <li>Let <code>l<\/code> be the first node and <code>r<\/code> be the second.<\/li>\n    <li>Call <code>swapPairs<\/code> recursively for the rest of the list starting from <code>r.next<\/code>.<\/li>\n    <li>Set <code>l.next<\/code> to the result of recursive call.<\/li>\n    <li>Make <code>r<\/code> point to <code>l<\/code> and return <code>r<\/code> as the new head.<\/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) due to recursive call stack.<\/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=\"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=\"java\">Java<\/button>\n    <button class=\"wp_blog_code-tab-button\" data-lang=\"py\">Python<\/button>\n    <button class=\"wp_blog_code-tab-button\" data-lang=\"csharp\">C#<\/button>\n  <\/div>\n\n  <div class=\"wp_blog_code-tab-content active\" data-lang=\"js\">\n    <pre><code class=\"language-javascript\">\nvar swapPairs = function(head) {\n    if (!head || !head.next) return head;\n\n    let l = head;\n    let r = head.next;\n\n    l.next = swapPairs(r.next);\n    r.next = l;\n    return r;\n};\n    <\/code><\/pre>\n  <\/div>\n\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"cpp\">\n    <pre><code class=\"language-cpp\">\nclass Solution {\npublic:\n    ListNode* swapPairs(ListNode* head) {\n        if (!head || !head->next) return head;\n\n        ListNode* first = head;\n        ListNode* second = head->next;\n\n        first->next = swapPairs(second->next);\n        second->next = first;\n\n        return second;\n    }\n};\n    <\/code><\/pre>\n  <\/div>\n\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"c\">\n    <pre><code class=\"language-c\">\nstruct ListNode* swapPairs(struct ListNode* head) {\n    if (!head || !head->next) return head;\n\n    struct ListNode* first = head;\n    struct ListNode* second = head->next;\n\n    first->next = swapPairs(second->next);\n    second->next = first;\n\n    return second;\n}\n    <\/code><\/pre>\n  <\/div>\n\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"java\">\n    <pre><code class=\"language-java\">\nclass Solution {\n    public ListNode swapPairs(ListNode head) {\n        if (head == null || head.next == null) return head;\n\n        ListNode first = head;\n        ListNode second = head.next;\n\n        first.next = swapPairs(second.next);\n        second.next = first;\n\n        return second;\n    }\n}\n    <\/code><\/pre>\n  <\/div>\n\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"py\">\n    <pre><code class=\"language-python\">\nclass Solution(object):\n    def swapPairs(self, head):\n        if not head or not head.next:\n            return head\n\n        first = head\n        second = head.next\n\n        first.next = self.swapPairs(second.next)\n        second.next = first\n\n        return second\n    <\/code><\/pre>\n  <\/div>\n\n  <div class=\"wp_blog_code-tab-content\" data-lang=\"csharp\">\n    <pre><code class=\"language-csharp\">\npublic class Solution {\n    public ListNode SwapPairs(ListNode head) {\n        if (head == null || head.next == null) return head;\n\n        ListNode first = head;\n        ListNode second = head.next;\n\n        first.next = SwapPairs(second.next);\n        second.next = first;\n\n        return second;\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    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   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You must solve the problem without modifying the values in the list&#8217;s nodes (i.e., only nodes themselves may be changed.) Examples: Input: head = [1,2,3,4]Output: [2,1,4,3] Input: head = []Output: [] Input: head = [1]Output: [1] Constraints: The number of<\/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":{"0":"post-6974","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-c-c-plus-plus","7":"category-csharp","8":"category-data-structures-and-algorithms","9":"category-dsa","10":"category-java","11":"category-javascript","12":"category-python"},"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/6974","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=6974"}],"version-history":[{"count":2,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/6974\/revisions"}],"predecessor-version":[{"id":6976,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/6974\/revisions\/6976"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=6974"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=6974"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=6974"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}