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Decode the Coding Interview in Swift: Real-World Examples

Feature #6: Identify a Species

Explore how to identify a species by finding the longest substring of unique nucleotides in a DNA sequence. Learn techniques to track nucleotide positions and apply sliding window logic to solve this problem efficiently in Swift.

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Description

The DNA for an alien species consists of a sequence of nucleotides. We can uniquely identify a species by finding the longest substring of nucleotides in the DNA, where no nucleotide appears twice. This substring of DNA is known as a species marker. Given an animal’s DNA, where each nucleotide is represented by a letter, we can find its species marker.

The following examples might clarify the requirements:

Solution

The basic idea is to traverse the entire sequence of nucleotides and search for a substring in which no nucleotides appear twice. For each visited nucleotide, we can store its last occurrence in a dictionary, with the key as the nucleotide and the value as its last position in the sequence.

Here’s how we implement this feature.

  1. We initialize the following set of variables to 0 to keep track of the visited nucleotides:

    • stCurr: the starting index of the current substring.
    • longest: the length of the longest substring.
    • start: the starting index of the longest substring.
    • currLen: the length of the current substring.

  2. For every nucleotide in the sequence, we check whether it is present in the dictionary or not.

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