London| 26 March SDC | Jamal Laqdiem| Sprint 2 | Improve code with precomputing

[jamallaqdiem]

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Changelist

In these two exercises I optimised performance for the common_prefix and count_letters tasks by applying pre-computing strategies.

[cjyuan]

Code is correct.

Can you use complexity to explain how the new implementation is better than the original implementation?

[jamallaqdiem]

@cjyuan thanks for reviewing my PR.
common_prefix:

Original: Used nested loops to compare every string combination. Time complexity was O(N^2 * M) (where N is the number of strings and M is string length)..
New: We sort the strings first and only compare adjacent items. Time complexity improved to O(N log N * M), which is much faster for large inputs.

count_letters:
Original: Looked up letters inside the string repeatedly, leading to extra work.
New: We do a single pass over the string to build two sets, then check for uppercase-only letters in O(1) constant time. Total time complexity is O(N).

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[cjyuan]

All good.

Note:

Original: Looked up letters inside the string repeatedly, leading to extra work.

"extra work" is ambiguous. Would be more precise to show complexity in big O notation.

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