From af048de87ea89a5d23d826bc89467b90dd51cbec Mon Sep 17 00:00:00 2001
From: JeevaRamanathan <jeevaramanathan.m@infosys.com>
Date: Wed, 1 Oct 2025 00:00:29 +0000
Subject: [PATCH 1/3] added isomorphic strings

Signed-off-by: JeevaRamanathan <jeevaramanathan.m@infosys.com>
---
 strings/is_isomorphic.py | 48 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 strings/is_isomorphic.py

diff --git a/strings/is_isomorphic.py b/strings/is_isomorphic.py
new file mode 100644
index 000000000000..244f4cb17f87
--- /dev/null
+++ b/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

From 634bf4ff5f4cde19b321aa39fc1379a91a6ed4e3 Mon Sep 17 00:00:00 2001
From: JeevaRamanathan <JeevaRamanathan@users.noreply.github.com>
Date: Wed, 1 Oct 2025 00:01:20 +0000
Subject: [PATCH 2/3] updating DIRECTORY.md

---
 DIRECTORY.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/DIRECTORY.md b/DIRECTORY.md
index 36acb3b97f1e..02bf5282280c 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -1333,6 +1333,7 @@
   * [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)

From 4049b24d67d31e7da0ee3f0b04457553cfb2834f Mon Sep 17 00:00:00 2001
From: Christian Clauss <cclauss@me.com>
Date: Thu, 17 Sep 2026 12:55:18 +0200
Subject: [PATCH 3/3] Update docstring with external references

Added links to Wikipedia and LeetCode for reference.
---
 strings/is_isomorphic.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/strings/is_isomorphic.py b/strings/is_isomorphic.py
index 244f4cb17f87..3e90e2d536eb 100644
--- a/strings/is_isomorphic.py
+++ b/strings/is_isomorphic.py
@@ -1,7 +1,7 @@
 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://en.wikipedia.org/wiki/Isomorphism
     https://leetcode.com/problems/isomorphic-strings/description/
 
     Two strings s and t are isomorphic if the characters in s can be