Two Sum

📌 Definition:

📖 Detailed Explanation:

Brute force is O(n^2). The optimal solution is a single pass with a hash map: for each number, check whether target - number was already seen; if so, return the stored index and the current index.

💻 Solutions:

• JavaScript

function twoSum(nums, target) {
  const seen = new Map(); // value -> index
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen.has(need)) return [seen.get(need), i];
    seen.set(nums[i], i);
  }
  return [];
}

🔑 Key Points:

• Hash map turns O(n^2) into O(n)
• Store value → index as you go
• Check complement BEFORE inserting current (avoids using an element twice)
• Handles duplicates correctly

🌍 Real-World Example: