Find the Missing Number

📌 Definition:

📖 Detailed Explanation:

The O(n) O(1)-space trick uses the sum formula: expected sum of 0..n is n*(n+1)/2; subtract the actual array sum to get the missing number. XOR is an overflow-safe alternative.

💻 Solutions:

• JavaScript

function missingNumber(nums) {
  const n = nums.length;
  const expected = (n * (n + 1)) / 2;
  const actual = nums.reduce((a, b) => a + b, 0);
  return expected - actual;
}

🔑 Key Points:

• Sum formula n*(n+1)/2 minus actual sum → missing
• O(n) time, O(1) space
• XOR approach avoids large-number overflow
• Works because numbers are distinct and from a known range

🌍 Real-World Example: