JavaScript DSA – Find the Missing Number in an Array

 🔍 Problem Statement:


> You are given an array containing n distinct numbers taken from 1 to n+1. One number is missing. Find it efficiently using JS.




💡 Solution:


function findMissingNumber(arr) {

  const n = arr.length + 1;

  const expectedSum = (n * (n + 1)) / 2;

  const actualSum = arr.reduce((acc, num) => acc + num, 0);

  return expectedSum - actualSum;

}

console.log(findMissingNumber([1, 2, 4, 5, 6])); // Output: 3


🧠 Explanation:


We know the sum of 1 to n is n*(n+1)/2.


Subtracting the actual sum from this gives the missing number.


This approach is better than nested loops or sorting.


---


✅ Bonus Tip:


> Use .reduce() in JS for summing values in arrays. It’s a commonly asked method in interviews!


---


✅ Try It Yourself:


What if 2 numbers are missing?


How to solve it without extra space?


---


📢 Engage:


Share your variation in the comments.


Follow the Code With Jaffer Blog for daily tips!




Comments

Post a Comment

Popular posts from this blog

Day 4 – Palindrome Number in JavaScript Without Converting to String

🔥 Interview Series – Day 4 💡 Topic: How to check if a number is a palindrome without converting it to a string in JavaScript ✅ Problem Statement Check whether a given number is a palindrome without converting it to a string. A number is a palindrome if it reads the same backward as forward. 🧪 Examples: 121 → true 123 → false 1221 → true -121 → false (negative numbers are not palindromes) 🧠 JavaScript Code (Math-Based Logic): function isPalindromeNumber(num) {   if (num < 0) return false;   let original = num;   let reversed = 0;   while (num > 0) {     reversed = reversed * 10 + (num % 10);     num = Math.floor(num / 10);   }   return original === reversed; } 💡 Tips to Remember: Avoid using .toString() — focus on mathematical logic. % (modulo) gives the last digit. Math.floor() removes the last digit from the number. Store the reversed number and compare it to the original. 🧑‍💻 Why Interviewers Ask This: To test pure logic-b...
Read more

Day 9 – Count Vowels in a String using JavaScript | DSA Challenge with Code & Explanation

  🧠 Day 9: Count Vowels in a String – JavaScript Interview Prep Welcome to Day 9 of the #CodeWithJaffer JavaScript DSA series! Today, we’ll solve a popular string-based interview problem: “Count the number of vowels in a given string.” 🔍 Problem Statement: Write a JavaScript function that takes a string as input and returns the count of vowels (a, e, i, o, u) present in it. ✅ Example Input & Output: // Input: "CodeWithJaffer" // Output: 5 vowels (o, e, i, a, e) 💡 Method 1: Using RegEx function countVowels(str) { return (str.match(/[aeiou]/gi) || []).length; } ✅ Explanation: /[aeiou]/gi matches all vowels (both uppercase and lowercase). match() returns an array of matches, or null if none. || [] ensures no error when match returns null. .length gives the count of vowels. 💡 Method 2: Using For-Loop function countVowelsLoop(str) { let count = 0; const vowels = "aeiouAEIOU"; for (let char of str) { if (vowels.i...
Read more

Day 13: Find Missing Number in an Array – JavaScript DSA Challenge

🔔 Day 13: Find Missing Number in an Array – JavaScript DSA Welcome to #Day13 of the CodeWithJaffer DSA series! 📌 Problem: Given an array containing numbers from 0 to n with one number missing, find that missing number. 🧪 Examples: Input: [3, 0, 1] Output: 2 Input: [0, 1, 2, 4, 5] Output: 3 ✅ JavaScript Code: function findMissingNumber(arr) { const n = arr.length; const total = (n * (n + 1)) / 2; const sum = arr.reduce((acc, curr) => acc + curr, 0); return total - sum; } console.log(findMissingNumber([3, 0, 1])); // 2 console.log(findMissingNumber([0, 1, 2, 4, 5])); // 3 💡 Tips: Use math formula for sum: n(n+1)/2 Reduce method calculates actual sum Subtract to get missing number 🎯 Bonus: Try solving with XOR approach too – it’s commonly asked in interviews! 🔗 Follow me for more: LinkedIn: www.linkedin.com/in/reach-jaffer YouTube: @codewithjaffer
Read more