Amazon Interview Question: We have numbers from 1 to 52 ... | Glassdoor

Interview Question

Software Engineer Interview Seattle, WA

We have numbers from 1 to 52 that are put into a 51 number

  array, what's the best way to find out which number is missing?

Interview Answer

4 Answers


my answer was to sort the array and use a boolean variable with a while loop that will exit when the number is not equal to the previous number plus 1

Interview Candidate on Jun 17, 2010

The above answer is a typical answer or brute force technique, which anyone could answer. What they are looking for is probably how good you are at math and algorithms. My answer would be:

1) Calculate the sum of all numbers stored in the array of size 51.
2) Subtract the sum from (52 * 53)/2 ---- Formula : n (n+1)/2.

The result of subtraction is the answer for this question.

Babji Chetty on Jun 19, 2010

Just want to add some comments to Babji Chetty's answer.

To calculate sum of complete sequence of 1 to 52 is used arithmetic progression sum formula. s = n(2a1+d(n-1))/2, where d is the step. Using this formula you can calculate sum of full sequence even if increment step not equal to 1.

Oleh on Jul 8, 2010

Here is the solution for finding all the missing numbers from a given array:

Bhumik on Oct 26, 2014

Add Answers or Comments

To comment on this, Sign In or Sign Up.