{"id":8290,"date":"2025-07-25T14:45:18","date_gmt":"2025-07-25T09:15:18","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=8290"},"modified":"2025-10-25T17:27:16","modified_gmt":"2025-10-25T11:57:16","slug":"postorder-iterative-approach-two-stack","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/postorder-iterative-approach-two-stack\/","title":{"rendered":"Postorder Iterative Approach Two Stack"},"content":{"rendered":"\n<!-- PostOrder - TwoStack -->\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: #030302;\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\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\n.wp_blog_explanation code {\n  background: #fef9f4; 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\/* makes it the reference for absolute children *\/\n}\n\n.wp_blog_toggle-btn {\n  position: absolute;\n  top: 1rem;\n  right: 1rem;\n  z-index: 9999;\n  padding: 0.5rem 0.8rem;\n  border-radius: 10%;\n  background: var(--primary);\n  color: white;\n  font-weight: bold;\n  cursor: pointer;\n  border: none;\n  box-shadow: var(--shadow);\n  transition: background 0.3s, transform 0.2s;\n}\n\n.wp_blog_toggle-btn:hover {\n  background: #cc772e;\n}\n\n.wp_blog_theme.dark-mode .wp_blog_code-tabs-container {\n  background: #1e1e1e;\n}\n<\/style>\n\n<div class=\"wp_blog_container wp_blog_theme\"> \n          <button id=\"blogNotesThemeToggle\" class=\"wp_blog_toggle-btn\">\ud83c\udf19<\/button>\n\n    <h1 class=\"wp_blog_main-heading\"><\/h1>\n    <div class=\"wp_blog_explanation\">\n        <h2>Problem Statement:<\/h2>\n\n        <p>Given the <code>root<\/code> of a binary tree, return <i>the postorder traversal of its nodes&#8217; values.<\/i>.\n        <\/p>\n                <h2>Examples:<\/h2>\n                <h3>Example 1:<\/h3>\n                <p><strong>Input:<\/strong> root = [1,null,2,3]<\/p>\n                <p><strong>Output:<\/strong> [3,2,1]<\/p>\n                <strong>Explanation:<\/strong>\n                <p>\n                  <img decoding=\"async\" src=\"https:\/\/namastedev.com\/blog\/wp-content\/uploads\/2025\/07\/Screenshot-2025-07-23-at-12.26.13\u202fPM.png\" alt=\"\">\n                <\/p>\n\n                <h3>Example 2:<\/h3>\n                <p><strong>Input:<\/strong> root = [1,2,3,4,5,null,8,null,null,6,7,9]<\/p>\n                <p><strong>Output:<\/strong> [4,6,7,5,2,9,8,3,1]<\/p>\n                <strong>Explanation:<\/strong>\n                <p>\n                  <img decoding=\"async\" src=\"https:\/\/namastedev.com\/blog\/wp-content\/uploads\/2025\/07\/Screenshot-2025-07-22-at-4.02.21\u202fPM.png\" alt=\"\">\n                <\/p>\n\n                <h3>Example 3:<\/h3>\n                <p><strong>Input:<\/strong> root = []<\/p>\n                <p><strong>Output:<\/strong> []<\/p>\n\n                <h3>Example 4:<\/h3>\n                <p><strong>Input:<\/strong> root = [1]<\/p>\n                <p><strong>Output:<\/strong> [1]<\/p>\n                \n                    <h2>Constraints:<\/h2>\n                    <ul>\n                      <li>The number of nodes in the tree is in the range <code>[0, 100]<\/code>.<\/li>\n                      <li><code>-100 <= Node.val <= 100<\/code><\/li>\n                    <\/ul>\n\n                <h2>Approach<\/h2>\n             <ul>\n                <li>Use two stacks: <code>s1<\/code> for processing nodes and <code>s2<\/code> for storing them in reverse postorder.<\/li>\n                <li>Push root to <code>s1<\/code>.<\/li>\n                <li>While <code>s1<\/code> is not empty:\n                <ul>\n\n                    <li>Pop from <code>s1<\/code>, push to <code>s2<\/code>.<\/li>\n                    <li>Push left child (if any) to <code>s1<\/code>.<\/li>\n                    <li>Push right child (if any) to <code>s1<\/code>.<\/li>\n                <\/ul>\n                \n                <\/li>\n\n                <li>After processing, pop from s2 and store node values in ans (this gives postorder).<\/li>\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(n)<\/strong><\/p>\n                <\/li>\n<h2>Dry Run<\/h2>\n<div style=\"background: var(--light-bg); border-left: 4px solid var(--primary); padding: 1rem; border-radius: var(--tab-radius); margin: 1rem 0; color: var(--text-dark);\">\n  <p><strong>Input:<\/strong> <code>root = [1, 2, 3, null, 4]<\/code><\/p>\n  <pre style=\"white-space: pre-wrap; background: var(--code-bg); padding: 1rem; border-radius: 8px; overflow-x: auto; color: var(--code-text);\">\n\nTree Structure:\n        1\n       \/ \\\n      2   3\n       \\\n        4\n\nInitialize:\ns1 = [1]\ns2 = []\nans = []\n\n--- First While Loop (s1 not empty) ---\n\n Pop 1 from s1 \u2192 curr = 1\n Push 1 to s2 \u2192 s2 = [1]\n Push 2 to s1 \u2192 s1 = [2]\n Push 3 to s1 \u2192 s1 = [2, 3]\n\n Pop 3 from s1 \u2192 curr = 3\n Push 3 to s2 \u2192 s2 = [1, 3]\n (No children to push)\n\n Pop 2 from s1 \u2192 curr = 2\n Push 2 to s2 \u2192 s2 = [1, 3, 2]\n Push 4 to s1 \u2192 s1 = [4]\n\n Pop 4 from s1 \u2192 curr = 4\n Push 4 to s2 \u2192 s2 = [1, 3, 2, 4]\n (No children to push)\n\n--- Second While Loop (s2 not empty) ---\n\n Pop 4 from s2 \u2192 ans = [4]\n Pop 2 from s2 \u2192 ans = [4, 2]\n Pop 3 from s2 \u2192 ans = [4, 2, 3]\n Pop 1 from s2 \u2192 ans = [4, 2, 3, 1]\n\nTraversal complete.\n\nFinal ans (postorder) = [4, 2, 3, 1]\n  <\/pre>\n\n  <p><strong>Output:<\/strong> <code>Result: [4, 2, 3, 1]<\/code><\/p>\n<\/div>\n\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 postorderTraversal = function(root) {\n    if(!root) return [];\n    let s1 = [root];\n    let s2 = [];\n\n    while(s1.length) {\n        let curr = s1.pop();\n        s2.push(curr);\n        curr.left && s1.push(curr.left);\n        curr.right && s1.push(curr.right);\n    }\n    let ans = [];\n    while(s2.length) {\n        ans.push(s2.pop().val);\n    }\n    return ans;\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\">\ndef postorderTraversal(root):\n    if not root:\n        return []\n    s1 = [root]\n    s2 = []\n\n    while s1:\n        curr = s1.pop()\n        s2.append(curr)\n        if curr.left:\n            s1.append(curr.left)\n        if curr.right:\n            s1.append(curr.right)\n\n    ans = []\n    while s2:\n        ans.append(s2.pop().val)\n    return ans\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<Integer> postorderTraversal(TreeNode root) {\n    if (root == null) return new ArrayList<>();\n    Stack<TreeNode> s1 = new Stack<>();\n    Stack<TreeNode> s2 = new Stack<>();\n    s1.push(root);\n\n    while (!s1.isEmpty()) {\n        TreeNode curr = s1.pop();\n        s2.push(curr);\n        if (curr.left != null) s1.push(curr.left);\n        if (curr.right != null) s1.push(curr.right);\n    }\n\n    List<Integer> ans = new ArrayList<>();\n    while (!s2.isEmpty()) {\n        ans.add(s2.pop().val);\n    }\n    return ans;\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<int> postorderTraversal(TreeNode* root) {\n    if (!root) return {};\n    stack<TreeNode*> s1, s2;\n    s1.push(root);\n    while (!s1.empty()) {\n        TreeNode* curr = s1.top(); s1.pop();\n        s2.push(curr);\n        if (curr->left) s1.push(curr->left);\n        if (curr->right) s1.push(curr->right);\n    }\n    vector<int> ans;\n    while (!s2.empty()) {\n        ans.push_back(s2.top()->val);\n        s2.pop();\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 postorderTraversal(struct TreeNode* root, int* result, int* returnSize) {\n    if (!root) return;\n\n    struct TreeNode* s1[1000];\n    struct TreeNode* s2[1000];\n    int top1 = -1, top2 = -1;\n    s1[++top1] = root;\n    while (top1 >= 0) {\n        struct TreeNode* curr = s1[top1--];\n        s2[++top2] = curr;\n        if (curr->left) s1[++top1] = curr->left;\n        if (curr->right) s1[++top1] = curr->right;\n    }\n\n    *returnSize = 0;\n    while (top2 >= 0) {\n        result[(*returnSize)++] = s2[top2--]->val;\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<int> PostorderTraversal(TreeNode root) {\n    if (root == null) return new List<int>();\n    Stack<TreeNode> s1 = new Stack<TreeNode>();\n    Stack<TreeNode> s2 = new Stack<TreeNode>();\n    s1.Push(root);\n\n    while (s1.Count > 0) {\n        TreeNode curr = s1.Pop();\n        s2.Push(curr);\n        if (curr.left != null) s1.Push(curr.left);\n        if (curr.right != null) s1.Push(curr.right);\n    }\n\n    List<int> ans = new List<int>();\n    while (s2.Count > 0) {\n        ans.Add(s2.Pop().val);\n    }\n    return ans;\n}\n       <\/code><\/pre>\n        <\/div>\n    <\/div>\n<\/div>\n\n<script>\ndocument.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) => content.classList.remove(\"active\"));\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  const themeToggle = document.getElementById(\"blogNotesThemeToggle\");\n  const themeContainer = document.querySelector(\".wp_blog_theme\");\n\n  themeToggle.addEventListener(\"click\", () => {\n    themeContainer.classList.toggle(\"dark-mode\");\n    themeToggle.textContent =\n      themeContainer.classList.contains(\"dark-mode\") ? \"\u2600\ufe0f\" : \"\ud83c\udf19\";\n  });\n});\n<\/script>\n","protected":false},"excerpt":{"rendered":"<p>\ud83c\udf19 Problem Statement: Given the root of a binary tree, return the postorder traversal of its nodes&#8217; values.. Examples: Example 1: Input: root = [1,null,2,3] Output: [3,2,1] Explanation: Example 2: Input: root = [1,2,3,4,5,null,8,null,null,6,7,9] Output: [4,6,7,5,2,9,8,3,1] Explanation: Example 3: Input: root = [] Output: [] Example 4: Input: root = [1] Output: [1] Constraints: The<\/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":[322,176,175,211,174,172,173],"tags":[],"class_list":["post-8290","post","type-post","status-publish","format-standard","category-algorithms-and-data-structures","category-csharp","category-cplusplus","category-data-structures","category-java","category-javascript","category-python"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8290","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=8290"}],"version-history":[{"count":2,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8290\/revisions"}],"predecessor-version":[{"id":10625,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8290\/revisions\/10625"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=8290"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=8290"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=8290"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}