feat: add isomorphic strings

DIRECTORY.md

@@ -1335,6 +1335,7 @@
   * [Indian Phone Validator](strings/indian_phone_validator.py)
   * [Is Contains Unique Chars](strings/is_contains_unique_chars.py)
   * [Is Isogram](strings/is_isogram.py)
+  * [Is Isomorphic](strings/is_isomorphic.py)
   * [Is Pangram](strings/is_pangram.py)
   * [Is Polish National Id](strings/is_polish_national_id.py)
   * [Is Spain National Id](strings/is_spain_national_id.py)

strings/is_isomorphic.py

@@ -0,0 +1,48 @@
+def is_isomorphic(s: str, t: str) -> bool:
+    """
+    LeetCode No. 205 Isomorphic Strings
+    Given two strings s and t, determine if they are isomorphic.
+    https://leetcode.com/problems/isomorphic-strings/description/
+
+    Two strings s and t are isomorphic if the characters in s can be
+    replaced to get t.
+
+    All occurrences of a character must be replaced with another character
+    while preserving the order of characters. No two characters may map to
+    the same character, but a character may map to itself.
+
+
+    >>> is_isomorphic("egg", "add")
+    True
+    >>> is_isomorphic("foo", "bar")
+    False
+    >>> is_isomorphic("paper", "title")
+    True
+    >>> is_isomorphic("ab", "aa")
+    False
+    """
+    if len(s) != len(t):
+        return False
+
+    mapping: dict[str, str] = {}
+    mapped = set()
+
+    for char_s, char_t in zip(s, t):
+        if char_s in mapping:
+            if mapping[char_s] != char_t:
+                return False
+        else:
+            if char_t in mapped:
+                return False
+            mapping[char_s] = char_t
+            mapped.add(char_t)
+
+    return True
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    print(is_isomorphic("egg", "add"))  # True