Add missing type annotations for strings directory (#5817)

* Type annotations for `strings/autocomplete_using_trie.py`

* Update autocomplete_using_trie.py

* Update detecting_english_programmatically.py

* Update detecting_english_programmatically.py

* Update frequency_finder.py

* Update frequency_finder.py

* Update frequency_finder.py

* Update word_occurrence.py

* Update frequency_finder.py

* Update z_function.py

* Update z_function.py

* Update frequency_finder.py

strings/autocomplete_using_trie.py

@@ -1,11 +1,13 @@
+from __future__ import annotations
+
 END = "#"
 
 
 class Trie:
-    def __init__(self):
-        self._trie = {}
+    def __init__(self) -> None:
+        self._trie: dict = {}
 
-    def insert_word(self, text):
+    def insert_word(self, text: str) -> None:
         trie = self._trie
         for char in text:
             if char not in trie:
@@ -13,7 +15,7 @@ class Trie:
             trie = trie[char]
         trie[END] = True
 
-    def find_word(self, prefix):
+    def find_word(self, prefix: str) -> tuple | list:
         trie = self._trie
         for char in prefix:
             if char in trie:
@@ -22,7 +24,7 @@ class Trie:
                 return []
         return self._elements(trie)
 
-    def _elements(self, d):
+    def _elements(self, d: dict) -> tuple:
         result = []
         for c, v in d.items():
             if c == END:
@@ -39,26 +41,28 @@ for word in words:
     trie.insert_word(word)
 
 
-def autocomplete_using_trie(s):
+def autocomplete_using_trie(string: str) -> tuple:
     """
     >>> trie = Trie()
     >>> for word in words:
     ...     trie.insert_word(word)
     ...
     >>> matches = autocomplete_using_trie("de")
-
-    "detergent " in matches
+    >>> "detergent " in matches
     True
-    "dog " in matches
+    >>> "dog " in matches
     False
     """
-    suffixes = trie.find_word(s)
-    return tuple(s + w for w in suffixes)
+    suffixes = trie.find_word(string)
+    return tuple(string + word for word in suffixes)
 
 
-def main():
+def main() -> None:
     print(autocomplete_using_trie("de"))
 
 
 if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
     main()