{"id":8316,"date":"2025-07-26T15:27:50","date_gmt":"2025-07-26T09:57:50","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=8316"},"modified":"2026-03-07T14:59:49","modified_gmt":"2026-03-07T09:29:49","slug":"postorderonestack","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/postorderonestack\/","title":{"rendered":"Post Order Iterative Approach One Stack"},"content":{"rendered":"\n<!-- PostOrder - OneStack -->\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: 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 <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 a stack to simulate <code>recursion<\/code>.<\/li>\n                <li>Traverse left subtree first, pushing <code>nodes<\/code> into the stack.<\/li>\n                <li><strong>Peek top node:<\/strong>\n                <ul>\n                    <li>If it has a right child <strong>not yet visited<\/strong>, traverse right subtree.<\/li>\n                    <li>Else, process the node (postorder: <code>left \u2192 right \u2192 root)<\/code>.<\/li>\n                <\/ul>\n                <\/li>\n                <li>Track the last visited node to avoid revisiting right subtrees.<\/li>\n                <li>Continue until all <code>nodes<\/code> are processed.<\/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(h)<\/strong> where: <code>h<\/code> where, h is the height of the tree.<\/p>\n                <\/li>\n\n<h2>Dry Run<\/h2>\n\n<div style=\"background: var(--light-bg); border-left: 4px solid var(--primary); padding: 1rem; border-radius: var(--tab-radius); margin: 1rem 0;\">\n\n<p><strong>Input:<\/strong> <code>root = [1, 2, 3, null, 4]<\/code><\/p>\n\n<pre style=\"white-space: pre-wrap; background: var(--code-bg); color: var(--code-text); padding: 1rem; border-radius: 8px; overflow-x: auto;\">\n\nTree Structure: \n        1\n       \/ \\\n      2   3\n       \\\n        4\n\nInitialize:\nstack = []\ncurr = 1\nans = []\nlastVisited = null\n\n--- First While Loop (curr || stack not empty) ---\n\ncurr = 1 \u2192 push to stack \u2192 stack = [1]\ncurr = 2 \u2192 push to stack \u2192 stack = [1, 2]\ncurr = null\n\npeek = 2\npeek.right = 4 and lastVisited != 4 \u2192 move curr = 4\n\ncurr = 4 \u2192 push to stack \u2192 stack = [1, 2, 4]\ncurr = null\n\npeek = 4\npeek.right is null \u2192 push 4 to ans \u2192 ans = [4]\npop 4 \u2192 lastVisited = 4\n\npeek = 2\npeek.right = 4 and lastVisited == 4 \u2192 push 2 to ans \u2192 ans = [4, 2]\npop 2 \u2192 lastVisited = 2\n\npeek = 1\npeek.right = 3 and lastVisited != 3 \u2192 move curr = 3\n\ncurr = 3 \u2192 push to stack \u2192 stack = [1, 3]\ncurr = null\n\npeek = 3\npeek.right is null \u2192 push 3 to ans \u2192 ans = [4, 2, 3]\npop 3 \u2192 lastVisited = 3\n\npeek = 1\npeek.right = 3 and lastVisited == 3 \u2192 push 1 to ans \u2192 ans = [4, 2, 3, 1]\npop 1 \u2192 lastVisited = 1\n\nstack is empty and curr is null \u2192 traversal complete\n\n<\/pre>\n\n<p><strong>Output:<\/strong> <code>Result: [4, 2, 3, 1]<\/code><\/p>\n\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 postorderTraversal = function(root) {\n    let stack = [];\n    let curr = root;\n    let ans = [];\n    let lastVisited = null;\n    while(curr || stack.length) {\n        while(curr) {\n            stack.push(curr);\n            curr = curr.left;\n        }\n        let peek = stack[stack.length - 1];\n        if(peek.right && peek.right != lastVisited){\n            curr = peek.right;\n        } else {\n            ans.push(peek.val);\n            lastVisited = stack.pop();\n        }\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    stack = []\n    curr = root\n    ans = []\n    lastVisited = None\n\n    while curr or stack:\n        while curr:\n            stack.append(curr)\n            curr = curr.left\n        peek = stack[-1]\n        if peek.right and peek.right != lastVisited:\n            curr = peek.right\n        else:\n            ans.append(peek.val)\n            lastVisited = stack.pop()\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    Stack<TreeNode> stack = new Stack<>();\n    TreeNode curr = root;\n    List<Integer> ans = new ArrayList<>();\n    TreeNode lastVisited = null;\n\n    while (curr != null || !stack.isEmpty()) {\n        while (curr != null) {\n            stack.push(curr);\n            curr = curr.left;\n        }\n        TreeNode peek = stack.peek();\n        if (peek.right != null && peek.right != lastVisited) {\n            curr = peek.right;\n        } else {\n            ans.add(peek.val);\n            lastVisited = stack.pop();\n        }\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    stack<TreeNode*> stack;\n    TreeNode* curr = root;\n    vector<int> ans;\n    TreeNode* lastVisited = nullptr;\n\n    while (curr || !stack.empty()) {\n        while (curr) {\n            stack.push(curr);\n            curr = curr->left;\n        }\n        TreeNode* peek = stack.top();\n        if (peek->right && peek->right != lastVisited) {\n            curr = peek->right;\n        } else {\n            ans.push_back(peek->val);\n            lastVisited = stack.top();\n            stack.pop();\n        }\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\">\n#include &lt;stdio.h&gt; \n#include &lt;stdlib.h&gt;\n#include &lt;stdbool.h&gt;   \n#define MAX 1000  \ntypedef struct TreeNode {\n    int val;\n    struct TreeNode* left;\n    struct TreeNode* right;\n} TreeNode;\ntypedef struct {\n    TreeNode* data[MAX];\n    int top;\n} Stack;\nvoid initStack(Stack* s) {\n    s->top = -1;\n}\nbool isEmpty(Stack* s) {\n    return s->top == -1;\n}\nvoid push(Stack* s, TreeNode* node) {\n    if (s->top < MAX - 1) {\n        s->data[++s->top] = node;\n    }\n}\nTreeNode* pop(Stack* s) {\n    if (!isEmpty(s)) {\n        return s->data[s->top--];\n    }\n    return NULL;\n}\nTreeNode* peek(Stack* s) {\n    if (!isEmpty(s)) {\n        return s->data[s->top];\n    }\n    return NULL;\n}\nvoid postorderTraversal(TreeNode* root) {\n    Stack stack;\n    initStack(&stack);\n    TreeNode* curr = root;\n    TreeNode* lastVisited = NULL;\n    while (curr || !isEmpty(&stack)) {\n        while (curr) {\n            push(&stack, curr);\n            curr = curr->left;\n        }\n        TreeNode* peekNode = peek(&stack);\n        if (peekNode->right && peekNode->right != lastVisited) {\n            curr = peekNode->right;\n        } else {\n            printf(\"%d \", peekNode->val);  \/\/ Output the node value\n            lastVisited = pop(&stack);\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    Stack<TreeNode> stack = new Stack<TreeNode>();\n    TreeNode curr = root;\n    List<int> ans = new List<int>();\n    TreeNode lastVisited = null;\n\n    while (curr != null || stack.Count > 0) {\n        while (curr != null) {\n            stack.Push(curr);\n            curr = curr.left;\n        }\n        TreeNode peek = stack.Peek();\n        if (peek.right != null && peek.right != lastVisited) {\n            curr = peek.right;\n        } else {\n            ans.Add(peek.val);\n            lastVisited = stack.Pop();\n        }\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 = 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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":[210,322,176,175,211,810,174,172,173],"tags":[],"class_list":["post-8316","post","type-post","status-publish","format-standard","category-algorithms","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\/8316","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=8316"}],"version-history":[{"count":3,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8316\/revisions"}],"predecessor-version":[{"id":11671,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/8316\/revisions\/11671"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=8316"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=8316"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=8316"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}