{"id":7673,"date":"2025-07-09T00:50:18","date_gmt":"2025-07-08T19:20:18","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=7673"},"modified":"2025-10-02T13:33:59","modified_gmt":"2025-10-02T08:03:59","slug":"usingstacks","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/usingstacks\/","title":{"rendered":"Implement Queue using Stacks"},"content":{"rendered":"\n<!-- using one stack -->\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<h1 class=\"wp_blog_main-heading\"><\/h1>\n\n<div class=\"wp_blog_explanation\">\n    <h2>Problem Statement:<\/h2>\n    <p>\n       Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (<code>push<\/code>, <code>peek<\/code>, <code>pop<\/code>, and <code>empty<\/code>).\n    <\/p>\n\n    <p>Implement the <code>MyQueue<\/code> class:<\/p>\n    <p>\n       <ul>\n        <li><code>void push(int x)<\/code>Pushes element x to the back of the queue.<\/li>\n        <li><code>int pop()<\/code>Removes the element from the front of the queue and returns it.<\/li>\n        <li><code>int peek()<\/code>Returns the element at the front of the queue.<\/li>\n        <li><code>boolean empty()<\/code>Returns <code>true<\/code> if the queue 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 stack, which means only <code>push to top<\/code>, <code>peek\/pop from top<\/code>, <code>size<\/code> and <code>is empty<\/code> operations are valid.<\/li>\n        <li>Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack&#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;MyQueue&#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, 1, 1, false]<\/code><\/p>\n    <p><strong>Explanation<\/strong>\n    <pre>\n    MyQueue myQueue = new MyQueue();\n    myQueue.push(1);    \/\/ queue is: [1]\n    myQueue.push(2);    \/\/ queue is: [1, 2] \n    (leftmost is front of the queue)\n    myQueue.peek();     \/\/ return 1\n    myQueue.pop();      \/\/ return 1, queue is [2]\n    myQueue.empty();    \/\/ return false\n    <\/pre>\n    <\/p>\n    <h2>Constraints:<\/h2>\n    <ul>\n        <li><code>1 <= x <= 9<\/code><\/li>\n        <li>At most <code>100<\/code> calls will be made to <code>push<\/code>, <code>pop<\/code>, <code>peek<\/code>, and <code>empty<\/code>.<\/li>\n        <li>All the calls to <code>pop<\/code> and <code>peek<\/code>are valid.<\/li>\n    <\/ul>\n\n    <p>\n        <strong>Follow-up<\/strong>:\n        Can you implement the queue such that each operation is <strong style=\"color: rgb(65, 81, 187);\">amortized<\/strong> <code>O(1)<\/code> time complexity? In other words, performing n operations will take overall <code>O(n)<\/code> time even if one of those operations may take longer.\n    <\/p>\n\n    <h2>Approach: Using Two Stacks (s1 & s2)<\/h2>\n    <ul>\n        <li>\n            Use two stacks, s1 and s2.\n        <\/li>\n\n        <li>\n            For Push, push onto s1. \n        <\/li>\n        <li>For pop, transfer elements from s1 to s2 if s2 is empty, then pop from s2.<\/li>\n        <li>For peek, transfer elements from s1 to s2 if s2 is empty, then return top of s2.<\/li>\n    <\/ul>\n\n    <h2>Visualisation:<\/h2>\n    <img decoding=\"async\"\n        src=\"https:\/\/namastedev.com\/blog\/wp-content\/uploads\/2025\/07\/Screenshot-2025-07-09-at-12.48.20\u202fAM.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 \u2192 O(n)<\/strong><\/p>\n                  <p><strong>peek \u2192 O(n)<\/strong><\/p>\n                    <\/li> \n                <h2>Space Complexity:<\/h2>\n                <li>\n                  <p><strong>Space Complexity = O(n)<\/strong> \n                <\/p>\n                <\/li>\n\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) peek() pop() peek() 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): s1 = [1] s2 = []\nAfter push(2):\ns1 = [1, 2]\ns2 = []\nAfter push(3):\ns1 = [1, 2, 3]\ns2 = []\npeek():\n\u2192 s2 is empty \u2192 move all from s1 to s2\ns1.pop() = 3 \u2192 s2.push(3)\ns1.pop() = 2 \u2192 s2.push(2)\ns1.pop() = 1 \u2192 s2.push(1)\nNow: s1 = [], s2 = [3, 2, 1]\nTop of s2 = 1\n\u2192 Returns: 1\npop():\n\u2192 s2 not empty \u2192 pop from s2\ns2.pop() = 1\nNow: s1 = [], s2 = [3, 2]\n\u2192 Returns: 1\npeek():\n\u2192 s2 not empty\nTop of s2 = 2\n\u2192 Returns: 2\nempty():\ns1 = [], s2 = [3, 2] \u2192 Not empty\n\u2192 Returns: false\n<\/pre>\n<p><strong>Final State:<\/strong> <code>s1 = []<\/code>, <code>s2 = [3, 2]<\/code> (top to bottom)<\/p> <\/div>\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<pre><code class=\"language-javascript\">\nvar MyQueue = function() {\n    this.s1 = [];\n    this.s2 = [];\n};\nMyQueue.prototype.push = function(x) {\n    this.s1.push(x);\n};\nMyQueue.prototype.pop = function() {\n    if(this.s2.length === 0) {\n        while(this.s1.length) {\n            this.s2.push(this.s1.pop());\n        }\n    }\n    return this.s2.pop();\n};\nMyQueue.prototype.peek = function() {\n        if(this.s2.length === 0) {\n        while(this.s1.length) {\n            this.s2.push(this.s1.pop());\n        }\n    }\n    return this.s2[this.s2.length-1];\n    \n};\nMyQueue.prototype.empty = function() {\n    return this.s1.length === 0 && this.s2.length === 0;\n};\n<\/code><\/pre>\n\n<\/div>\n\n    <div class=\"wp_blog_code-tab-content\" data-lang=\"py\">\n        <pre><code class=\"language-python\">\nclass MyQueue:\n    def __init__(self):\n        self.s1 = []\n        self.s2 = []\n\n    def push(self, x: int) -> None:\n        self.s1.append(x)\n\n    def pop(self) -> int:\n        if not self.s2:\n            while self.s1:\n                self.s2.append(self.s1.pop())\n        return self.s2.pop()\n\n    def peek(self) -> int:\n        if not self.s2:\n            while self.s1:\n                self.s2.append(self.s1.pop())\n        return self.s2[-1]\n\n    def empty(self) -> bool:\n        return not self.s1 and not self.s2\n    <\/code><\/pre>\n    <\/div>\n    <div class=\"wp_blog_code-tab-content\" data-lang=\"java\">\n        <pre><code class=\"language-java\">\nimport java.util.Stack;\n\nclass MyQueue {\n    Stack<Integer> s1 = new Stack<>();\n    Stack<Integer> s2 = new Stack<>();\n    public void push(int x) {\n        s1.push(x);\n    }\n    public int pop() {\n        if (s2.isEmpty()) {\n            while (!s1.isEmpty()) {\n                s2.push(s1.pop());\n            }\n        }\n        return s2.pop();\n    }\n    public int peek() {\n        if (s2.isEmpty()) {\n            while (!s1.isEmpty()) {\n                s2.push(s1.pop());\n            }\n        }\n        return s2.peek();\n    }\n    public boolean empty() {\n        return s1.isEmpty() && s2.isEmpty();\n    }\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;stack&gt;\nclass MyQueue {\n    std::stack<int> s1, s2;\npublic:\n    void push(int x) {\n        s1.push(x);\n    }\n    int pop() {\n     if (s2.empty()) {\n       while (!s1.empty()) {\n          s2.push(s1.top());\n          s1.pop();\n         }\n      }\n       int val = s2.top();\n       s2.pop();\n       return val;\n    }\n\n    int peek() {\n      if (s2.empty()) {\n        while (!s1.empty()) {\n             s2.push(s1.top());\n             s1.pop();\n            }\n        }\n        return s2.top();\n    }\n    bool empty() {\n        return s1.empty() && s2.empty();\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 s1[MAX], s2[MAX];\n    int top1, top2;\n} MyQueue;\nvoid initQueue(MyQueue* q) {\n    q->top1 = -1;\n    q->top2 = -1;\n}\nvoid push(MyQueue* q, int x) {\n    q->s1[++(q->top1)] = x;\n}\nint pop(MyQueue* q) {\n    if (q->top2 == -1) {\n        while (q->top1 >= 0) {\n            q->s2[++(q->top2)] = q->s1[(q->top1)--];\n        }\n    }\n    return q->s2[(q->top2)--];\n}\nint peek(MyQueue* q) {\n    if (q->top2 == -1) {\n        while (q->top1 >= 0) {\n            q->s2[++(q->top2)] = q->s1[(q->top1)--];\n        }\n    }\n    return q->s2[q->top2];\n}\nbool empty(MyQueue* q) {\n    return q->top1 == -1 && q->top2 == -1;\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;\npublic class MyQueue {\n    private Stack<int> s1 = new Stack<int>();\n    private Stack<int> s2 = new Stack<int>();\n\n    public void Push(int x) {\n        s1.Push(x);\n    }\n    public int Pop() {\n        if (s2.Count == 0) {\n            while (s1.Count > 0) {\n                s2.Push(s1.Pop());\n            }\n        }\n        return s2.Pop();\n    }\n    public int Peek() {\n        if (s2.Count == 0) {\n            while (s1.Count > 0) {\n                s2.Push(s1.Pop());\n            }\n        }\n        return s2.Peek();\n    }\n    public bool Empty() {\n        return s1.Count == 0 && s2.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 first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty). Implement the MyQueue class: void push(int x)Pushes element x to the back of the queue. int pop()Removes the element from the front of<\/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":{"0":"post-7673","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-algorithms-and-data-structures","7":"category-c-c-plus-plus","8":"category-csharp","9":"category-cplusplus","10":"category-data-structures","11":"category-data-structures-and-algorithms","12":"category-dsa","13":"category-java","14":"category-javascript","15":"category-python"},"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/7673","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=7673"}],"version-history":[{"count":6,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/7673\/revisions"}],"predecessor-version":[{"id":10210,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/7673\/revisions\/10210"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=7673"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=7673"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=7673"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}