Day 28EasyPractice

The Missing Number

Sum Formula / XOR

Problem

Logic:

Sum: ExpectedSum - ActualSum = Missing
XOR: (1 oplus 2 dots oplus n) oplus (arr[0] oplus dots oplus arr[n-1])

Solve it yourself before you look

Reading the solution first feels like progress, but it makes the next similar problem — and the interview version — much harder, because you skipped the part where you figure it out. Give it an honest 20–30 minutes. Stuck? Re-read the pattern, watch the concept video, or try the brute force first.

Hidden: approach · solution code

Revising this one?

→Solve it from memory before peeking
→Say the pattern out loud, not just the code
→Explain it like you would to a friend
→Try it again in a different language