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In the divide-and-conquer algorithm for maximum sum subarray, the maximum subarray may lie:

  1. entirely in the left half
     
  2. entirely in the right half

What is the third possible case?

  1. It must contain only positive elements.
     
  2. It must be exactly the middle element.
     
  3. It crosses the middle, using a suffix of the left half and a prefix of the right half.
     
  4. It must start at the first element and end at the last element.

1 Answer

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In divide and conquer for maximum sum subarray, the array is divided into two halves.

The maximum sum subarray can occur in three possible ways.

Case $1$: It lies completely in the left half.

Case $2$: It lies completely in the right half.

Case $3$: It crosses the middle.

If it crosses the middle, then it must include:

  • a suffix of the left half
     
  • a prefix of the right half

So, the crossing sum is calculated by taking the best suffix sum from the left side and the best prefix sum from the right side.

Therefore, the third possible case is:

It crosses the middle, using a suffix of the left half and a prefix of the right half.

Answer: C

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