Create longest-palindromic-substring.py

strings/longest-palindromic-substring.py

@@ -0,0 +1,23 @@
+def longest_palindromic_substring(s: str) -> str:
+    if (
+        len(s) < 2 or s == s[::-1]
+    ):  # Quick check for a single character or an already palindromic string
+        return s
+
+    start, max_len = 0, 1
+
+    for i in range(1, len(s)):
+        # Odd-length palindrome (centered at s[i])
+        odd = s[i - max_len - 1 : i + 1]
+        # Even-length palindrome (centered between s[i-1] and s[i])
+        even = s[i - max_len : i + 1]
+
+        # Check which palindrome is longer
+        if i - max_len - 1 >= 0 and odd == odd[::-1]:
+            start = i - max_len - 1
+            max_len += 2
+        elif even == even[::-1]:
+            start = i - max_len
+            max_len += 1
+
+    return s[start : start + max_len]