Easy Arrays Question 7 of 224

How do you find the missing number in an array of n distinct values from 0 to n?

DSA interview set · Speak this in 60–90 seconds · Faridabad & Delhi NCR

01234

Index starts at 0. Scan once for max — O(n).

The expected sum is n*(n+1)/2

Why interviewers care about Arrays:

This is a process

question about Arrays.

Panels listen for order,

trade-offs, and what you would actually do on a DSA project - not buzzwords.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

End with when you use it and one common pitfall.

Before you speak the answer, walk the interviewer through these steps:

1
Define it
The expected sum is n*(n+1)/2
2
subtract the actual sum
to get the missing value in O(n) time and O(1) space.
3
XOR of all indices
and values also works and avoids overflow.
4
A hash set is
O(n) space and is the fallback if you forget the math.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

Here's a short line you can speak, broken into clear beats:

Say this line

“XOR of all indices and values also works and avoids overflow.”

Break into beats

XORofallindicesandvalues

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

The expected sum is n*(n+1)/2 subtract the actual sum to get the missing value in O(n) time and O(1) space.