Why should you care?

Interviewers ask Big O on every solution. Production capacity planning starts here too.

See it live — copy this example

Run snippets in a .NET console app, LINQPad, or https://dotnetfiddle.net. Write Big O above every solution.

// O(n)
int Sum(int[] a) {
  int s = 0;
  foreach (var x in a) s += x;
  return s;
}
// O(n²)
bool HasDuplicateBrute(int[] a) {
  for (int i = 0; i < a.Length; i++)
    for (int j = i + 1; j < a.Length; j++)
      if (a[i] == a[j]) return true;
  return false;
}

What happened?

• One loop over n items is O(n).
• Nested loops over n are O(n²).
• Dictionary can make duplicate checks O(n).

Practice next

1. Classify for/foreach nesting in a past solution.
2. Rewrite HasDuplicateBrute with HashSet.
3. Compare Big O of both.
4. Add an O(log n) binary search example.
5. Plot n=10 vs n=10_000 mentally.

Remember

Big O ignores constants. Nested loops multiply. Better structures change the class.

Interview opener

“What is the complexity?”

Outcome: You answer with n and the dominant term.