feat:longest common prefix in a string

[AmbrishRamachandiran]

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Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

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[appgurueu]

Code-wise fine except for the repeated string concatenation; this is usually optimized out in practice, but here it's def. just cleaner to keep track of max. index up to which all strings share a common prefix, and then use that to substring shortestString.

(Algorithm-wise still pretty trivial, though.)