{"id":9858,"date":"2025-09-01T14:48:14","date_gmt":"2025-09-01T09:18:14","guid":{"rendered":"https:\/\/namastedev.com\/blog\/?p=9858"},"modified":"2026-04-17T17:55:00","modified_gmt":"2026-04-17T12:25:00","slug":"single_number","status":"publish","type":"post","link":"https:\/\/namastedev.com\/blog\/single_number\/","title":{"rendered":"\u00a0Single Number"},"content":{"rendered":"\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  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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 a <strong>non-empty<\/strong> array of integers <code>nums<\/code>, every element appears twice except for one. <strong>Find that single one<\/strong>.\n        <\/p>\n\n        <p>\n          <strong>You must implement a solution with a linear runtime complexity and use only constant extra space.\n            <\/strong>\n          <\/p>\n                <h2>Examples:<\/h2>\n\n                <h4>Example 1:<\/h4>\n                <p><strong>Input:<\/strong> nums = [2, 2, 1]<\/p>\n                <p><strong>Output:<\/strong> 1<\/p>\n\n                <h4>Example 2:<\/h4>\n                <p><strong>Input:<\/strong> nums = [4, 1, 2, 1, 2]<\/p>\n                <p><strong>Output:<\/strong> 4<\/p>\n\n                <h4>Example 3:<\/h4>\n                <p><strong>Input:<\/strong> nums = [1]<\/p>\n                <p><strong>Output:<\/strong> 1<\/p>\n                \n\n                    <h2>Constraints:<\/h2>\n                    <ul>\n                      <li><code>1 \u2264 nums.length \u2264 3 \u00d7 10<sup>4<\/sup><\/code><\/li>\n                      <li><code>-3 \u00d7 10<sup>4<\/sup> \u2264 nums[i] \u2264 3 \u00d7 10<sup>4<\/sup><\/code><\/li>\n                      <li><code>Each element appears twice except one that appears only once.<\/code><\/li>\n                    <\/ul>\n\n                <h2>Approach 1 (Brute-force Hash Map):<\/h2>\n                <ul>\n                    <li><strong>Create an empty<\/strong> <code>hash map<\/code> to store counts of each element.<\/li>\n                    <li>Loop through the <code>array<\/code>, <strong>update the count<\/strong> for each element.<\/li>\n                    <li>Loop again to <strong>find the element<\/strong> with count <code>1<\/code> and return it.<\/li>\n                <\/ul>\n\n                <h2>Time Complexity:<\/h2>\n                <li>\n                  <p><strong>Time Complexity = O(n)<\/strong>\n                We traverse the array twice: once for counting and once for checking.\n                  <\/li>\n                <h2>Space Complexity:<\/h2>\n                <li>\n                  <p><strong>Space Complexity = O(n)<\/strong>\n                  The hash map may store counts for up to <code>n<\/code> elements in the worst case.<\/p>\n                <\/li>\n\n<h2>Dry Run<\/h2>\n  <div style=\"background: #f9f9f9; border-left: 4px solid var(--primary); padding: 1rem; border-radius: var(--tab-radius); margin: 1rem 0;\">\n  <p><strong>Input:<\/strong> <code>nums = [4, 2, 1, 0, 5]<\/code><\/p>\n  <pre style=\"white-space: pre-wrap; background: #fff5ea; padding: 1rem; border-radius: 8px; overflow-x: auto;\">\nStep 1: Initialize hash = {}\n\nFirst loop (counting occurrences):\n    i = 0 \u2192 nums[0] = 4 \u2192 not in hash \u2192 hash = {4: 1}\n    i = 1 \u2192 nums[1] = 2 \u2192 not in hash \u2192 hash = {4: 1, 2: 1}\n    i = 2 \u2192 nums[2] = 1 \u2192 not in hash \u2192 hash = {4: 1, 2: 1, 1: 1}\n    i = 3 \u2192 nums[3] = 0 \u2192 not in hash \u2192 hash = {4: 1, 2: 1, 1: 1, 0: 1}\n    i = 4 \u2192 nums[4] = 5 \u2192 not in hash \u2192 hash = {4: 1, 2: 1, 1: 1, 0: 1, 5: 1}\n\nAfter first loop: hash = {4: 1, 2: 1, 1: 1, 0: 1, 5: 1}\n\nSecond loop (finding element with count = 1):\n    i = 0 \u2192 nums[0] = 4 \u2192 hash[4] = 1 \u2192 return 4\n\nFinal return = 4\n  <\/pre>\n  <p><strong>Output:<\/strong> <code>4<\/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-2025-07-12-at-9.57.58\u202fAM.png\" alt=\"Stocks\" \/>\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\">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 singleNumber = function(nums) {\n    let hash = {};\n    for (let i = 0; i < nums.length; i++) {\n        if (!hash[nums[i]]) {\n            hash[nums[i]] = 1;\n        } else {\n            hash[nums[i]]++;\n        }\n    }\n    for (let i = 0; i < nums.length; i++) {\n        if (hash[nums[i]] === 1) {\n            return nums[i];\n        }\n    }\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\">\nclass Solution:\n    def singleNumber(self, nums):\n        hash = {}\n        for num in nums:\n            hash[num] = hash.get(num, 0) + 1\n        for num in nums:\n            if hash[num] == 1:\n                return num\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\">\nimport java.util.HashMap;\n\npublic class Solution {\n    public int singleNumber(int[] nums) {\n        HashMap<Integer, Integer> hash = new HashMap<>();\n        for (int num : nums) {\n            hash.put(num, hash.getOrDefault(num, 0) + 1);\n        }\n        for (int num : nums) {\n            if (hash.get(num) == 1) {\n                return num;\n            }\n        }\n        return -1;\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\">\n#include &lt;vector&gt;\n#include &lt;unordered_map&gt;\nusing namespace std;\n\nclass Solution {\npublic:\n    int singleNumber(vector<int>& nums) {\n        unordered_map<int, int> hash;\n        for (int num : nums) {\n            hash[num]++;\n        }\n        for (int num : nums) {\n            if (hash[num] == 1)\n                return num;\n        }\n        return -1;\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 <stdlib.h> -->\n  #include &lt;stdio.h&gt;\nint singleNumber(int* nums, int numsSize) {\n    for (int i = 0; i < numsSize; i++) {\n        int count = 0;\n        for (int j = 0; j < numsSize; j++) {\n            if (nums[j] == nums[i]) count++;\n        }\n        if (count == 1) return nums[i];\n    }\n    return -1;\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\">\nusing System;\nusing System.Collections.Generic;\npublic class Solution {\n    public int SingleNumber(int[] nums) {\n        Dictionary<int, int> hash = new Dictionary<int, int>();\n        for (int i = 0; i < nums.Length; i++) {\n            if (!hash.ContainsKey(nums[i])) {\n                hash[nums[i]] = 1;\n            } else {\n                hash[nums[i]]++;\n            }\n        }\n        for (int i = 0; i < nums.Length; i++) {\n            if (hash[nums[i]] == 1) {\n                return nums[i];\n            }\n        }\n        return -1; \n    }\n}     <\/code><\/pre>\n        <\/div>\n    <\/div>\n<\/div>\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\n                <h2>Approach 2 (Optimal using XOR):<\/h2>\n                <ul>\n                    <li><code>XOR<\/code> of two <strong>same numbers<\/strong> is 0: <code>a ^ a = 0<\/code>.<\/li>\n                    <li><code>XOR<\/code> of a number with <code>0<\/code> is the number itself: <code>a ^ 0 = a<\/code><\/li>\n                    <li>So, if all elements occur <strong>twice except one<\/strong>, XOR-ing all gives that <strong>unique number<\/strong>.<\/li>\n                  <\/ul>\n\n                <h2>Time Complexity:<\/h2>\n                <li>\n                  <p><strong>Time Complexity = O(n)<\/strong>\n                where n is the number of elements in the array.\n                  <\/li>\n                <h2>Space Complexity:<\/h2>\n                <li>\n                  <p><strong>Space Complexity = O(1)<\/strong>\n                  No extra space used.<\/p>\n                <\/li>\n\n<h2>Dry Run<\/h2>\n  <div style=\"background: #f9f9f9; border-left: 4px solid var(--primary); padding: 1rem; border-radius: var(--tab-radius); margin: 1rem 0;\">\n  <p><strong>Input:<\/strong> <code>nums = [4, 2, 1, 0, 5]<\/code><\/p>\n  <pre style=\"white-space: pre-wrap; background: #fff5ea; padding: 1rem; border-radius: 8px; overflow-x: auto;\">\nStep 1: Initialize xor = 0\n\nLoop through array:\n    i = 0 \u2192 xor = 0 ^ 4 = 4\n    i = 1 \u2192 xor = 4 ^ 2 = 6\n    i = 2 \u2192 xor = 6 ^ 1 = 7\n    i = 3 \u2192 xor = 7 ^ 0 = 7\n    i = 4 \u2192 xor = 7 ^ 5 = 2\n\nFinal xor = 2\n  <\/pre>\n  <p><strong>Output:<\/strong> <code>2<\/code><\/p>\n  <\/div>\n        \n        <!-- <h2>Visualisation:<\/h2>\n        <img decoding=\"async\" src=\"https:\/\/namastedev.com\/blog\/wp-content\/uploads\/2025\/08\/Screenshot-2025-08-30-at-6.17.20\u202fPM.png\" alt=\"Stocks\" \/> -->\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\">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\">\n  var singleNumber = function(nums) {\n      let xor = 0;\n      for (let i = 0; i < nums.length; i++) {\n          xor = xor ^ nums[i];\n      }\n      return xor;\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\">\n class Solution:\n      def singleNumber(self, nums):\n          xor = 0\n          for num in nums:\n              xor ^= num\n          return xor\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\">\n  public class Solution {\n      public int singleNumber(int[] nums) {\n          int xor = 0;\n          for (int num : nums) {\n              xor ^= num;\n          }\n          return xor;\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\">\n<!-- #include &lt;algorithm&gt; -->\n class Solution {\n  public:\n      int singleNumber(vector& nums) {\n          int xorVal = 0;\n          for (int num : nums) {\n              xorVal ^= num;\n          }\n          return xorVal;\n      }\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 int singleNumber(int* nums, int numsSize) {\n      int xor = 0;\n      for (int i = 0; i < numsSize; i++) {\n          xor ^= nums[i];\n      }\n      return xor;\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\">\nusing System;\npublic class Solution {\n    public int SingleNumber(int[] nums) {\n        int xor = 0;\n        for (int i = 0; i < nums.Length; i++) {\n            xor = xor ^ nums[i];\n        }\n        return xor;\n    }\n}      <\/code><\/pre>\n        <\/div>\n    <\/div>\n<\/div>\n\n<script>\ndocument.addEventListener(\"DOMContentLoaded\", () => {\n    const containers = document.querySelectorAll(\".wp_blog_code-tabs-container\");\n\n    containers.forEach((container) => {\n        const buttons = container.querySelectorAll(\".wp_blog_code-tab-button\");\n        const contents = container.querySelectorAll(\".wp_blog_code-tab-content\");\n\n        buttons.forEach((button) => {\n            button.addEventListener(\"click\", () => {\n                const lang = button.getAttribute(\"data-lang\");\n\n                \/\/ Reset only inside this container\n                buttons.forEach((btn) => btn.classList.remove(\"active\"));\n                contents.forEach((content) => content.classList.remove(\"active\"));\n\n                \/\/ Activate selected tab\n                button.classList.add(\"active\");\n                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Find that single one. You must implement a solution with a linear runtime complexity and use only constant extra space. Examples: Example 1: Input: nums = [2, 2, 1] Output: 1 Example 2: Input: nums = [4, 1, 2,<\/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":[210,322,212,211,811,810,174,172,173],"tags":[],"class_list":["post-9858","post","type-post","status-publish","format-standard","category-algorithms","category-algorithms-and-data-structures","category-code-optimization","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\/9858","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=9858"}],"version-history":[{"count":3,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/9858\/revisions"}],"predecessor-version":[{"id":12214,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/posts\/9858\/revisions\/12214"}],"wp:attachment":[{"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/media?parent=9858"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/categories?post=9858"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namastedev.com\/blog\/wp-json\/wp\/v2\/tags?post=9858"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}