{"id":8834,"date":"2025-08-01T14:03:00","date_gmt":"2025-08-01T08:33:00","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=8834"},"modified":"2025-08-01T14:04:13","modified_gmt":"2025-08-01T08:34:13","slug":"zigzag-level-order-traversal","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/zigzag-level-order-traversal\/","title":{"rendered":"Zigzag Level Order Traversal"},"content":{"rendered":"\n<!-- Binary Tree Zigzag Level Order Traversal -->\n<link\n    href=\"https:\/\/cdn.jsdelivr.net\/npm\/prismjs@1.29.0\/themes\/prism-tomorrow.min.css\"\n    rel=\"stylesheet\"\n\/>\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_theme {\n  --primary: #E58C32;\n  --secondary: #f8c291;\n  --light-bg: #fef9f4;\n  --text-dark: #2d2d2d;\n  --tab-radius: 12px;\n  --shadow: 0 4px 12px rgba(0, 0, 0, 0.08);\n  --code-bg: #001f3f;\n  --code-text: #d4f1ff;\n}\n\n.wp_blog_container {\n  font-family: 'Segoe UI', sans-serif;\n  background: var(--light-bg);\n  margin: 0;\n  padding: 0;\n  color: var(--text-dark);\n}\n\n\/* Heading *\/\n.wp_blog_main-heading {\n  text-align: center;\n  font-size: 2.4rem;\n  color: var(--primary);\n  margin-top: 2.5rem;\n  font-weight: bold;\n}\n\n\/* Explanation Card *\/\n.wp_blog_explanation,\n.wp_blog_code-tabs-container {\n  max-width: 940px;\n  margin: 2rem auto;\n  padding: 2rem;\n  background: white;\n  border-radius: var(--tab-radius);\n  box-shadow: var(--shadow);\n}\n\n\/* Text and Visuals *\/\n.wp_blog_explanation h2{\n  font-size: 1.4rem;\n  color: var(--primary);\n  margin-bottom: 0.5rem;\n}\n.wp_blog_explanation h5{\n  color: var(--primary);\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}\n.wp_blog_explanation code {\n  background: #f9cea6;\n  color: #2d2d2d;\n  padding: 3px 6px;\n  border-radius: 4px;\n  font-family: 'Courier New', monospace;\n}\n.wp_blog_explanation img {\n  max-width: 100%;\n  border-radius: var(--tab-radius);\n  margin-top: 1rem;\n  box-shadow: 0 2px 12px rgba(0,0,0,0.06);\n}\n\n\/* Tab Buttons *\/\n.wp_blog_code-tabs-header {\n  display: flex;\n  flex-wrap: wrap;\n  gap: 0.5rem;\n  margin-bottom: 1rem;\n}\n.wp_blog_code-tab-button {\n  padding: 0.6rem 1.2rem;\n  border: 1px solid var(--primary);\n  background: white;\n  color: var(--primary);\n  border-radius: 50px;\n  font-weight: 600;\n  cursor: pointer;\n  transition: all 0.3s ease;\n}\n.wp_blog_code-tab-button:hover {\n  background: var(--secondary);\n}\n.wp_blog_code-tab-button.active {\n  background: var(--primary);\n  color: white;\n}\n\n\/* Code Content *\/\n.wp_blog_code-tab-content {\n  display: none;\n  background: var(--code-bg);\n  border-radius: var(--tab-radius);\n}\n.wp_blog_code-tab-content.active {\n  display: block;\n}\n.wp_blog_code-tab-content pre {\n  margin: 0;\n  padding: 1.5rem;\n  font-size: 1rem;\n  overflow-x: auto;\n  background: var(--code-bg);\n  border-radius: var(--tab-radius);\n  color: var(--code-text);\n}\n<\/style>\n\n<div class=\"wp_blog_container wp_blog_theme\"> \n    <h1 class=\"wp_blog_main-heading\"><\/h1>\n    <div class=\"wp_blog_explanation\">\n        <h2>Problem Statement:<\/h2>\n        <p>\n            Given the root of a binary tree, return the <i>zigzag level order traversal of its nodes&#8217; values.<\/i> (i.e., from left to right, then right to left for the next level and alternate between).\n        <\/p>\n       <h2>Examples:<\/h2>\n\n                <h3>Example 1:<\/h3>\n                <img decoding=\"async\" src=\"https:\/\/namastedev.com\/blog\/wp-content\/uploads\/2025\/08\/Screenshot-2025-08-01-at-12.56.03\u202fPM.png\" alt=\"\">\n                <p><strong>Input:<\/strong> root = [3,9,20,null,null,15,7]<\/p>\n                <p><strong>Output:<\/strong> [[3],[20,9],[15,7]]<\/p>\n\n                <h3>Example 2:<\/h3>\n                <p><strong>Input:<\/strong> root = [1]<\/p>\n                <p><strong>Output:<\/strong> [[1]]<\/p>\n\n                <h3>Example 3:<\/h3>\n                <p><strong>Input:<\/strong> root = []<\/p>\n                <p><strong>Output:<\/strong> []<\/p>\n\n                    <h2>Constraints:<\/h2>\n                    <ul>\n                        <li>The number of nodes in the tree is in the range <code>[0, 2000]<\/code>.<\/li>\n                        <li><code>-100 <= Node.val <= 100<\/code><\/li>\n                    <\/ul>\n\n                <h2>Approach<\/h2>\n                <ul>\n                    <li><strong>Use a Queue<\/strong> (q) for level-order traversal (BFS).<\/li>\n                    <li><strong>Track the current level<\/strong> (level) to know the direction:\n                    <ul>\n                        <li>Even levels: <strong>Left to Right<\/strong> \u2192 use <code>push()<\/code>.<\/li>\n                        <li>Odd levels: <strong>Right to Left<\/strong> \u2192 use <code>unshift()<\/code> to insert at the beginning.<\/li>\n                    <\/ul>\n                    <\/li>\n                    <li>For each level:\n                        <ul>\n                            <li>Process all nodes in the current level (<code>levelSize<\/code>)<\/li>\n                            <li>Collect their values in <code>levelArr<\/code>.<\/li>\n                            <li>Add their children to the queue.<\/li>\n                        <\/ul> \n                    <\/li>\n                    <li>After processing the level, push <code>levelArr<\/code> to <code>ans<\/code> and increase <code>level<\/code>.<\/li>\n                    <li>Return the final result array <code>ans<\/code>.<\/li>\n                    \n                <\/ul>\n\n                <h2>Time Complexity:<\/h2>\n                <li>\n                  <p><strong>Time Complexity = O(n)<\/strong>\n                  <\/li>\n                <h2>Space Complexity:<\/h2>\n                <li>\n                  <p><strong>Space Complexity = O(h)<\/strong> recursion stack space (h=tree height)<\/p>\n                <\/li>\n<h2>Dry Run<\/h2> <div style=\"background: #f9f9f9; padding: 10px; border: 1px solid #ccc; font-family: monospace;\"> <b>Function Call:<\/b> zigzagLevelOrder(root) where<br> root = TreeNode(1)<br> \u251c\u2500\u2500 left: TreeNode(2)<br> \u2502\u00a0\u00a0 \u251c\u2500\u2500 left: TreeNode(4)<br> \u2502\u00a0\u00a0 \u2514\u2500\u2500 right: TreeNode(5)<br> \u2514\u2500\u2500 right: TreeNode(3)<br><br>\n<b>Step 1:<\/b><br>\n\u2192 Check if root is null \u2192 No<br>\n\u2192 Initialize: ans = [], q = [1], level = 0<br><br>\n<b>Step 2 (level = 0):<\/b><br>\n\u2192 levelSize = 1<br>\n\u2192 Initialize levelArr = []<br>\n\u2192 i = 0: curr = 1<br>\n\u2192 level % 2 == 0 \u2192 Push 1 to levelArr \u2192 levelArr = [1]<br>\n\u2192 Push 2 and 3 to queue \u2192 q = [2, 3]<br>\n\u2192 Push levelArr to ans \u2192 ans = [[1]]<br>\n\u2192 Increment level \u2192 level = 1<br><br>\n<b>Step 3 (level = 1):<\/b><br>\n\u2192 levelSize = 2<br>\n\u2192 Initialize levelArr = []<br>\n\u2192 i = 0: curr = 2<br>\n\u2192 level % 2 != 0 \u2192 Unshift 2 \u2192 levelArr = [2]<br>\n\u2192 Push 4 and 5 to queue \u2192 q = [3, 4, 5]<br>\n\u2192 i = 1: curr = 3<br>\n\u2192 level % 2 != 0 \u2192 Unshift 3 \u2192 levelArr = [3, 2]<br>\n\u2192 No children to push<br>\n\u2192 Push levelArr to ans \u2192 ans = [[1], [3, 2]]<br>\n\u2192 Increment level \u2192 level = 2<br><br>\n<b>Step 4 (level = 2):<\/b><br>\n\u2192 levelSize = 2<br>\n\u2192 Initialize levelArr = []<br>\n\u2192 i = 0: curr = 4<br>\n\u2192 level % 2 == 0 \u2192 Push 4 \u2192 levelArr = [4]<br>\n\u2192 No children to push<br>\n\u2192 i = 1: curr = 5<br>\n\u2192 level % 2 == 0 \u2192 Push 5 \u2192 levelArr = [4, 5]<br>\n\u2192 No children to push<br>\n\u2192 q = [] \u2192 Queue is empty<br>\n\u2192 Push levelArr to ans \u2192 ans = [[1], [3, 2], [4, 5]]<br><br>\n<b>Final Result:<\/b> [[1], [3, 2], [4, 5]]\n<\/div>\n\n    <!-- <h2>Visualisation:<\/h2>\n        <img decoding=\"async\" src=\"https:\/\/namastedev.com\/blog\/wp-content\/uploads\/2025\/07\/Screenshot-2025-07-19-at-5.23.43\u202fPM.png\" alt=\"longest\" \/> -->\n    <\/div>\n    \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 zigzagLevelOrder = function(root) {\n    if (!root) return [];\n    let ans = [];\n    let q = [root];\n    let level = 0;\n    while (q.length) {\n        let levelArr = [];\n        let levelSize = q.length;\n        for (let i = 0; i < levelSize; i++) {\n            let curr = q.shift();\n            if (level % 2 === 0) {\n                levelArr.push(curr.val);\n            } else {\n                levelArr.unshift(curr.val);\n            }\n            if (curr.left) q.push(curr.left);\n            if (curr.right) q.push(curr.right);\n        }\n        ans.push(levelArr);\n        level++;\n    }\n    return ans;\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\">\nfrom collections import deque\n\ndef zigzagLevelOrder(root):\n    if not root:\n        return []\n    res = []\n    q = deque([root])\n    level = 0\n    while q:\n        size = len(q)\n        level_nodes = deque()\n        for _ in range(size):\n            node = q.popleft()\n            if level % 2 == 0:\n                level_nodes.append(node.val)\n            else:\n                level_nodes.appendleft(node.val)\n            if node.left:\n                q.append(node.left)\n            if node.right:\n                q.append(node.right)\n        res.append(list(level_nodes))\n        level += 1\n    return res\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 List<List<Integer>> zigzagLevelOrder(TreeNode root) {\n    List<List<Integer>> result = new ArrayList<>();\n    if (root == null) return result;\n    Queue<TreeNode> q = new LinkedList<>();\n    q.offer(root);\n    boolean leftToRight = true;\n\n    while (!q.isEmpty()) {\n        int size = q.size();\n        Integer[] level = new Integer[size];\n        for (int i = 0; i < size; i++) {\n            TreeNode curr = q.poll();\n            int index = leftToRight ? i : size - 1 - i;\n            level[index] = curr.val;\n\n            if (curr.left != null) q.offer(curr.left);\n            if (curr.right != null) q.offer(curr.right);\n        }\n        result.add(Arrays.asList(level));\n        leftToRight = !leftToRight;\n    }\n    return result;\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\">\nvector<vector<int>> zigzagLevelOrder(TreeNode* root) {\n    vector<vector<int>> ans;\n    if (!root) return ans;\n    queue<TreeNode*> q;\n    q.push(root);\n    bool leftToRight = true;\n\n    while (!q.empty()) {\n        int size = q.size();\n        vector<int> level(size);\n        for (int i = 0; i < size; ++i) {\n            TreeNode* node = q.front(); q.pop();\n            int index = leftToRight ? i : size - i - 1;\n            level[index] = node->val;\n\n            if (node->left) q.push(node->left);\n            if (node->right) q.push(node->right);\n        }\n        ans.push_back(level);\n        leftToRight = !leftToRight;\n    }\n    return ans;\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\">\nvoid zigzagLevelOrder(TreeNode* root) {\n    if (root == NULL) return;\n\n    Queue* q = createQueue();\n    enqueue(q, root);\n    int level = 0;\n\n    while (!isEmpty(q)) {\n        int size = queueSize(q);\n        int* levelArr = new int[size];\n\n        for (int i = 0; i < size; i++) {\n            TreeNode* curr = dequeue(q);\n            int index = (level % 2 == 0) ? i : size - 1 - i;\n            levelArr[index] = curr->val;\n\n            if (curr->left) enqueue(q, curr->left);\n            if (curr->right) enqueue(q, curr->right);\n        }\n        printArray(levelArr, size);\n        level++;\n    }\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 IList<IList<int>> ZigzagLevelOrder(TreeNode root) {\n    IList<IList<int>> result = new List<IList<int>>();\n    if (root == null) return result;\n\n    Queue<TreeNode> q = new Queue<TreeNode>();\n    q.Enqueue(root);\n    bool leftToRight = true;\n\n    while (q.Count > 0) {\n        int size = q.Count;\n        int[] level = new int[size];\n        for (int i = 0; i < size; i++) {\n            TreeNode curr = q.Dequeue();\n            int index = leftToRight ? i : size - i - 1;\n            level[index] = curr.val;\n            if (curr.left != null) q.Enqueue(curr.left);\n            if (curr.right != null) q.Enqueue(curr.right);\n        }\n        result.Add(level.ToList());\n        leftToRight = !leftToRight;\n    }\n    return result;\n}\n            <\/code><\/pre>\n        <\/div>\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\n                buttons.forEach((btn) => btn.classList.remove(\"active\"));\n                contents.forEach((content) =>\n                    content.classList.remove(\"active\")\n                );\n\n                button.classList.add(\"active\");\n                document\n                    .querySelector(`.wp_blog_code-tab-content[data-lang=\"${lang}\"]`)\n                    .classList.add(\"active\");\n            });\n        });\n    });\n<\/script>\n","protected":false},"excerpt":{"rendered":"<p>Problem Statement: Given the root of a binary tree, return the zigzag level order traversal of its nodes&#8217; values. (i.e., from left to right, then right to left for the next level and alternate between). Examples: Example 1: Input: root = [3,9,20,null,null,15,7] Output: [[3],[20,9],[15,7]] Example 2: Input: root = [1] Output: [[1]] Example 3: Input:<\/p>\n","protected":false},"author":108,"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":[322,176,175,211,810,174,172,173],"tags":[],"class_list":["post-8834","post","type-post","status-publish","format-standard","category-algorithms-and-data-structures","category-csharp","category-cplusplus","category-data-structures","category-dsa","category-java","category-javascript","category-python"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8834","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\/108"}],"replies":[{"embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/comments?post=8834"}],"version-history":[{"count":1,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8834\/revisions"}],"predecessor-version":[{"id":8835,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8834\/revisions\/8835"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=8834"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=8834"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=8834"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}