Day 8: Find Duplicate Number in Array – JavaScript Interview Question

 📌 Problem:

> Given an array of integers, find any duplicate number.

🧪 Example:

Input: [1, 2, 3, 4, 2]

Output: 2

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✅ Solution 1 – Using Set:

function findDuplicate(arr) {

  const seen = new Set();

  for (let num of arr) {

    if (seen.has(num)) return num;

    seen.add(num);

  }

  return -1;

}

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✅ Solution 2 – Sort & Compare:

function findDuplicate(arr) {

  arr.sort();

  for (let i = 1; i < arr.length; i++) {

    if (arr[i] === arr[i - 1]) return arr[i];

  }

  return -1;

}

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💡 Tips to Remember:

Set is great for uniqueness checks

Sorting reduces space, but increases time: O(n log n)

Use return -1 for "no duplicate" fallback