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One algorithm to rule them all

A fine day I came across an algorithmic challenge during an interview. Suppose you have a list of contiguous numbers, but it is somehow missing out on one number in the list. For an example, take this: 4,5,6,7,8,9,10,11,12,14,15 The task was to find the missing number which in this case is 13. I started with : parse through the list take the starting number take the last number take the sum of all the numbers in the list (sumA) = 101 calculate a sum using Arithmetic Progression summation formula n*(n+1)/2 (sumB) = 114 subtract the sum sumA from sumB you get the missing number

May 2, 2025 - 13:09
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One algorithm to rule them all

A fine day I came across an algorithmic challenge during an interview.

Suppose you have a list of contiguous numbers, but it is somehow missing out on one number in the list. For an example, take this:

4,5,6,7,8,9,10,11,12,14,15

The task was to find the missing number which in this case is 13.

I started with :

  • parse through the list
  • take the starting number
  • take the last number
  • take the sum of all the numbers in the list (sumA) = 101 calculate a sum using Arithmetic Progression summation formula n*(n+1)/2 (sumB) = 114
  • subtract the sum sumA from sumB
  • you get the missing number
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