{"id":7585,"date":"2025-07-05T19:17:37","date_gmt":"2025-07-05T13:47:37","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=7585"},"modified":"2025-10-02T13:18:29","modified_gmt":"2025-10-02T07:48:29","slug":"implement-stack-using-two-queues","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/implement-stack-using-two-queues\/","title":{"rendered":"Implement Stack using Two Queues"},"content":{"rendered":"\n<!-- PrismJS for Syntax Highlighting -->\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;   \/* light bg instead of dark blue *\/\n  color: #E58C32;        \/* brand orange *\/\n  padding: 3px 6px;\n  border-radius: 4px;\n  font-family: 'Courier New', monospace;\n  font-weight: 600;      \/* optional, makes it pop *\/\n}\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\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\n.wp_blog_code-tab-button:hover {\n  background: var(--secondary);\n}\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\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  background: var(--code-bg);\n  border-radius: var(--tab-radius);\n  color: var(--code-text);\n}\n\n\/* Dark mode variables *\/\n.wp_blog_theme.dark-mode {\n  --light-bg: #121212;\n  --text-dark: #f5f5f5;\n  --shadow: 0 4px 12px rgba(255, 255, 255, 0.08);\n  --code-bg: #1e1e1e;\n  --code-text: #c5f0ff;\n}\n\n.wp_blog_theme.dark-mode .wp_blog_explanation {\n  background: #1e1e1e;\n}\n\n\/* Dark mode code highlight *\/\n.wp_blog_theme.dark-mode .wp_blog_explanation code {\n  background: #333;\n  color: #ffd27f;\n}\n\n.wp_blog_theme {\n  position: relative; \/* 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\n<div class=\"wp_blog_explanation\">\n    <h2>Problem Statement:<\/h2>\n    <p>\n       Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (<code>push<\/code>, <code>top<\/code>, <code>pop<\/code>, and <code>empty<\/code>).\n    <\/p>\n\n    <p>Implement the <code>MyStack<\/code> class:<\/p>\n    <p>\n       <ul>\n        <li><code>void push(int x)<\/code>Pushes element x to the top of the stack.<\/li>\n        <li><code>int pop()<\/code>Removes the element on the top of the stack and returns it..<\/li>\n        <li><code>int top()<\/code>Returns the element on the top of the stack.<\/li>\n        <li><code>boolean empty()<\/code>Returns true if the stack is empty, <code>false<\/code> otherwise.<\/li>\n       <\/ul>\n    <\/p>\n\n    <p><strong>Notes<\/strong>\n    <ul>\n        <li>You must use only standard operations of a queue, which means that only <code>push to back<\/code>, <code>peek\/pop from front<\/code>, <code>size<\/code> and <code>is empty<\/code> operations are valid.<\/li>\n        <li>Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue&#8217;s standard operations.<\/li>\n    <\/ul>\n    <\/p>\n    <h2>Examples<\/h2>\n    <p><strong>Example 1:<\/strong><\/p>\n    <p><strong>Input:<\/strong>[&#8220;MyStack&#8221;, &#8220;push&#8221;, &#8220;push&#8221;, &#8220;top&#8221;, &#8220;pop&#8221;, &#8220;empty&#8221;]\n    [[], [1], [2], [], [], []]<\/p>\n    <p><strong>Output:<\/strong><code>[null, null, null, 2, 2, false]<\/code><\/p>\n    <p><strong>Explanation<\/strong>\n    MyStack myStack = new MyStack();\n    myStack.push(1);\n    myStack.push(2);\n    myStack.top(); \/\/ return 2\n    myStack.pop(); \/\/ return 2\n    myStack.empty(); \/\/ return False\n    <\/p>\n    <h2>Constraints:<\/h2>\n    <ul>\n        <li>1 <= x <= 9<\/li>\n        <li>At most <code>100<\/code> calls will be made to <code>push<\/code>, <code>pop<\/code>, <code>top<\/code>, and <code>empty<\/code>.<\/li>\n        <li>All the calls to <code>pop<\/code> and <code>top<\/code>are valid.<\/li>\n    <\/ul>\n\n    <p>\n        <strong>Follow-up<\/strong>:\n        Can you implement the stack using only one queue?\n    <\/p>\n\n    <h2>Approach<\/h2>\n    <ul>\n        <li>\n            Initialize Two Queues.\n        <\/li>\n\n        <li>\n            Just add the element to q1. (q1.push(x))\n        <\/li>\n\n        <li>\n            Remove the last pushed element (i.e., rear of q1).\n        <\/li>\n\n        <li>\n          Similar to pop(), but instead of removing the last element.\n        <\/li>\n        <li>\n          Check if q1 is empty.\n        <\/li>\n    <\/ul>\n\n    <h2>Visualisation:<\/h2>\n    <img decoding=\"async\"\n        src=\"https:\/\/namastedev.com\/blog\/wp-content\/uploads\/2025\/07\/Screenshot-2025-07-05-at-7.15.53\u202fPM.png\"\n        alt=\"stack\"\n    \/>\n                <h2>Time Complexity:<\/h2>\n                <li>\n                  <p><strong>push  \u2192 O(1)<\/strong><\/p>\n                  <p><strong>pop\/top  \u2192 O(n)<\/strong><\/p>\n                  <p><strong>empty  \u2192 O(1)<\/strong><\/p>\n                    <\/li> \n                <h2>Space Complexity:<\/h2>\n                <li>\n                  <p><strong>Space Complexity = O(1)<\/strong>\n                <li>In-place modifications with constant extra space.<\/li>  \n                <\/p>\n                <\/li>\n<h2>Dry Run<\/h2> <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);\"> <p><strong>Input:<\/strong><\/p> <pre style=\"white-space: pre-wrap; background: var(--code-bg); padding: 1rem; border-radius: 8px; overflow-x: auto; color: var(--code-text);\"> Operations: push(1) push(2) push(3) top() pop() top() empty() <\/pre> <p><strong>State Transitions:<\/strong><\/p> <pre style=\"white-space: pre-wrap; background: var(--code-bg); padding: 1rem; border-radius: 8px; overflow-x: auto; color: var(--code-text);\"> After push(1): q1 = [1] q2 = []\nAfter push(2):\nq1 = [1, 2]\nq2 = []\nAfter push(3):\nq1 = [1, 2, 3]\nq2 = []\ntop():\nMove q1 \u2192 q2 (except last):\nq2 = [1, 2]\nq1 = [3]\nPeek front = 3, push it to q2\nq2 = [1, 2, 3]\nSwap q1 and q2\nq1 = [1, 2, 3]\nq2 = []\n\u2192 Returns: <strong>3<\/strong>\npop():\nMove q1 \u2192 q2 (except last):\nq2 = [1, 2]\nq1 = [3]\nRemove last from q1: 3\nSwap q1 and q2\nq1 = [1, 2]\nq2 = []\n\u2192 Returns: <strong>3<\/strong>\ntop():\nMove q1 \u2192 q2 (except last):\nq2 = [1]\nq1 = [2]\nPeek front = 2, push it to q2\nq2 = [1, 2]\nSwap q1 and q2\nq1 = [1, 2]\nq2 = []\n\u2192 Returns: <strong>2<\/strong>\nempty():\nq1 = [1, 2] \u2192 Not empty\n\u2192 Returns: <strong>false<\/strong>\n<\/pre>\n<p><strong>Final State:<\/strong> <code>q1 = [1, 2]<\/code>, <code>Stack (top to bottom): [2, 1]<\/code><\/p> <\/div>\n\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\">\n            JavaScript\n        <\/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    <div class=\"wp_blog_code-tab-content active\" data-lang=\"js\">\n       \n<pre><code class=\"language-javascript\">\nvar MyStack = function() {\n  this.q1 = [];\n  this.q2 = [];\n};\n\nMyStack.prototype.push = function(x) {\n  this.q1.push(x);\n};\n\nMyStack.prototype.pop = function() {\n  let n = this.q1.length;\n  for (let i = 0; i &lt; n - 1; i++) {\n    this.q2.push(this.q1.shift());\n  }\n  let ans = this.q1.shift();\n  let temp = this.q1;\n  this.q1 = this.q2;\n  this.q2 = temp;\n  return ans;\n};\n\nMyStack.prototype.top = function() {\n  let n = this.q1.length;\n  for (let i = 0; i &lt; n - 1; i++) {\n    this.q2.push(this.q1.shift());\n  }\n  let front = this.q1[0];\n  this.q2.push(this.q1.shift());\n  let temp = this.q1;\n  this.q1 = this.q2;\n  this.q2 = temp;\n  return front;\n};\n\nMyStack.prototype.empty = function() {\n  return this.q1.length === 0;\n};\n<\/code><\/pre>\n\n<\/div>\n    <div class=\"wp_blog_code-tab-content\" data-lang=\"py\">\n        <pre><code class=\"language-python\">\nfrom collections import deque\n\nclass MyStack:\n\n    def __init__(self):\n        self.q1 = deque()\n        self.q2 = deque()\n\n    def push(self, x: int) -> None:\n        self.q1.append(x)\n\n    def pop(self) -> int:\n        while len(self.q1) > 1:\n            self.q2.append(self.q1.popleft())\n        ans = self.q1.popleft()\n        self.q1, self.q2 = self.q2, self.q1\n        return ans\n\n    def top(self) -> int:\n        while len(self.q1) > 1:\n            self.q2.append(self.q1.popleft())\n        front = self.q1.popleft()\n        self.q2.append(front)\n        self.q1, self.q2 = self.q2, self.q1\n        return front\n\n    def empty(self) -> bool:\n        return len(self.q1) == 0\n    <\/code><\/pre>\n    <\/div>\n    <div class=\"wp_blog_code-tab-content\" data-lang=\"java\">\n        <pre><code class=\"language-java\">\nclass MyStack {\n    Queue<Integer> q1 = new LinkedList<>();\n    Queue<Integer> q2 = new LinkedList<>();\n\n    public MyStack() {}\n\n    public void push(int x) {\n        q1.offer(x);\n    }\n\n    public int pop() {\n        while (q1.size() > 1) {\n            q2.offer(q1.poll());\n        }\n        int ans = q1.poll();\n        Queue<Integer> temp = q1;\n        q1 = q2;\n        q2 = temp;\n        return ans;\n    }\n\n    public int top() {\n        while (q1.size() > 1) {\n            q2.offer(q1.poll());\n        }\n        int front = q1.poll();\n        q2.offer(front);\n        Queue<Integer> temp = q1;\n        q1 = q2;\n        q2 = temp;\n        return front;\n    }\n\n    public boolean empty() {\n        return q1.isEmpty();\n    }\n} <\/code><\/pre>\n    <\/div>\n    <div class=\"wp_blog_code-tab-content\" data-lang=\"cpp\">\n        <pre><code class=\"language-cpp\">\n#include &lt;queue&gt;\nusing namespace std;\n\nclass MyStack {\n    queue<int> q1, q2;\n\npublic:\n    MyStack() {}\n\n    void push(int x) {\n        q1.push(x);\n    }\n\n    int pop() {\n        while (q1.size() > 1) {\n            q2.push(q1.front());\n            q1.pop();\n        }\n        int ans = q1.front(); q1.pop();\n        swap(q1, q2);\n        return ans;\n    }\n\n    int top() {\n        while (q1.size() > 1) {\n            q2.push(q1.front());\n            q1.pop();\n        }\n        int front = q1.front(); q1.pop();\n        q2.push(front);\n        swap(q1, q2);\n        return front;\n    }\n\n    bool empty() {\n        return q1.empty();\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\">\n#include &lt;stdio.h&gt;\n#include &lt;stdbool.h&gt;\n#define MAX 1000\n\ntypedef struct {\n    int q1[MAX];\n    int q2[MAX];\n    int front1, rear1;\n    int front2, rear2;\n} MyStack;\n\nMyStack* myStackCreate() {\n    MyStack* stack = (MyStack*)malloc(sizeof(MyStack));\n    stack->front1 = stack->rear1 = 0;\n    stack->front2 = stack->rear2 = 0;\n    return stack;\n}\n\nvoid push(MyStack* obj, int x) {\n    obj->q1[obj->rear1++] = x;\n}\n\nint pop(MyStack* obj) {\n    while (obj->rear1 - obj->front1 > 1) {\n        obj->q2[obj->rear2++] = obj->q1[obj->front1++];\n    }\n    int val = obj->q1[obj->front1++];\n    for (int i = 0; i < obj->rear2; i++) {\n        obj->q1[i] = obj->q2[i];\n    }\n    obj->rear1 = obj->rear2;\n    obj->front1 = obj->front2 = obj->rear2 = 0;\n    return val;\n}\n\nint top(MyStack* obj) {\n    while (obj->rear1 - obj->front1 > 1) {\n        obj->q2[obj->rear2++] = obj->q1[obj->front1++];\n    }\n    int val = obj->q1[obj->front1++];\n    obj->q2[obj->rear2++] = val;\n\n    for (int i = 0; i < obj->rear2; i++) {\n        obj->q1[i] = obj->q2[i];\n    }\n    obj->rear1 = obj->rear2;\n    obj->front1 = obj->front2 = obj->rear2 = 0;\n    return val;\n}\n\nbool empty(MyStack* obj) {\n    return obj->rear1 == obj->front1;\n}\n\nvoid myStackFree(MyStack* obj) {\n    free(obj);\n}\n    <\/code><\/pre>\n    <\/div>\n    <div class=\"wp_blog_code-tab-content\" data-lang=\"cs\">\n        <pre><code class=\"language-csharp\">\nusing System.Collections.Generic;\n\npublic class MyStack {\n    private Queue<int> q1 = new Queue<int>();\n    private Queue<int> q2 = new Queue<int>();\n\n    public MyStack() {}\n\n    public void Push(int x) {\n        q1.Enqueue(x);\n    }\n\n    public int Pop() {\n        while (q1.Count > 1) {\n            q2.Enqueue(q1.Dequeue());\n        }\n        int val = q1.Dequeue();\n        var temp = q1;\n        q1 = q2;\n        q2 = temp;\n        return val;\n    }\n\n    public int Top() {\n        while (q1.Count > 1) {\n            q2.Enqueue(q1.Dequeue());\n        }\n        int front = q1.Dequeue();\n        q2.Enqueue(front);\n        var temp = q1;\n        q1 = q2;\n        q2 = temp;\n        return front;\n    }\n\n    public bool Empty() {\n        return q1.Count == 0;\n    }\n}\n  <\/code><\/pre>\n    <\/div>\n<\/div>\n<\/div>\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: Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty). Implement the MyStack class: void push(int x)Pushes element x to the top of the stack. int pop()Removes the element on the top of the stack and<\/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":[322,260,176,175,211,811,810,174,172,173],"tags":[],"class_list":["post-7585","post","type-post","status-publish","format-standard","category-algorithms-and-data-structures","category-c-c-plus-plus","category-csharp","category-cplusplus","category-data-structures","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\/7585","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=7585"}],"version-history":[{"count":2,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/7585\/revisions"}],"predecessor-version":[{"id":10207,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/7585\/revisions\/10207"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=7585"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=7585"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=7585"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}