feat(strings): add longest common prefix algorithm

strings/longest_common_prefix.py

@@ -0,0 +1,77 @@
+"""
+Longest Common Prefix
+
+Find the longest string that is shared as a prefix by every string in a list.
+
+The list is sorted first and only the lexicographically smallest and the
+largest string are compared: a prefix shared by the whole list is necessarily
+a prefix of both extremes, so no other pair has to be examined.
+
+Reference: https://en.wikipedia.org/wiki/Longest_common_prefix
+"""
+
+
+def longest_common_prefix_of_two(first: str, second: str) -> str:
+    """
+    Return the longest prefix shared by ``first`` and ``second``.
+
+    >>> longest_common_prefix_of_two("flower", "flow")
+    'flow'
+    >>> longest_common_prefix_of_two("abc", "xyz")
+    ''
+    >>> longest_common_prefix_of_two("", "anything")
+    ''
+    >>> longest_common_prefix_of_two("same", "same")
+    'same'
+    """
+    index = 0
+    shortest_length = min(len(first), len(second))
+    while index < shortest_length and first[index] == second[index]:
+        index += 1
+    return first[:index]
+
+
+def longest_common_prefix(strings: list[str]) -> str:
+    """
+    Return the longest string that is a prefix of every element of ``strings``.
+
+    An empty list, or a list whose strings diverge at the first character,
+    yields the empty string.
+
+    >>> longest_common_prefix(["flower", "flow", "flight"])
+    'fl'
+    >>> longest_common_prefix(["dog", "racecar", "car"])
+    ''
+    >>> longest_common_prefix(["interview"])
+    'interview'
+    >>> longest_common_prefix(["", "banana"])
+    ''
+    >>> longest_common_prefix(["prefix", "prefix", "prefix"])
+    'prefix'
+    >>> longest_common_prefix(["ab", "abc", "abcd"])
+    'ab'
+    >>> longest_common_prefix([])
+    ''
+
+    A non-string element is rejected:
+
+    >>> longest_common_prefix(["flow", 42])
+    Traceback (most recent call last):
+        ...
+    ValueError: all elements of `strings` must be strings
+    """
+    for word in strings:
+        if not isinstance(word, str):
+            raise ValueError("all elements of `strings` must be strings")
+
+    if not strings:
+        return ""
+
+    ordered = sorted(strings)
+    return longest_common_prefix_of_two(ordered[0], ordered[-1])
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()