Longest palindromic string

Author: ratulb

codes/longest_palidromic_substr.mojo

@@ -0,0 +1,42 @@
+### Longest Palindromic Substring
+### Given a string s, return the longest palindromic substring therein
+
+
+fn longest_palindrome(s: String) -> String:
+    if len(s) == 0:
+        return s
+    longest = String("")
+    for i in range(len(s)):
+        left, right = i, i
+        find_longest(left, right, s, longest)
+        left, right = i, i + 1
+        find_longest(left, right, s, longest)
+    return longest
+
+
+fn find_longest(mut left: Int, mut right: Int, s: String, mut longest: String):
+    while 0 <= left and right < len(s) and s[left] == s[right]:
+        if right - left + 1 > len(longest):
+            longest = s[left : right + 1]
+        left -= 1
+        right += 1
+
+
+from testing import assert_true
+
+
+fn main() raises:
+    var s: String = "babad"
+    var expected: String = "bab"
+    result = longest_palindrome(s)
+    assert_true(result == expected, "Assertion failed")
+
+    s = "cbbd"
+    expected = "bb"
+    result = longest_palindrome(s)
+    assert_true(result == expected, "Assertion failed")
+
+    s = "racecar"
+    expected = "racecar"
+    result = longest_palindrome(s)
+    assert_true(result == expected, "Assertion failed")

docs/index.md

@@ -52,3 +52,5 @@
 🟢 [Longest Common Subsequence Recursive](longest_common_subeq.md) ➔ Return the length of the longest common subsequence between two strings, or 0 if none exists
 
 🟢 [Longest Common Subsequence Dynamic](longest_common_subeq_dyn.md) ➔ Return the length of the longest common subsequence between two strings, or 0 if none exists
+
+🟢 [Longest Palindromic Substring](longest_palidromic_substr.md) ➔ Given a string s, return the longest palindromic substring therein

docs/longest_palidromic_substr.md

@@ -0,0 +1,49 @@
+### Longest Palindromic Substring
+### Given a string s, return the longest palindromic substring therein
+
+```python
+
+
+fn longest_palindrome(s: String) -> String:
+    if len(s) == 0:
+        return s
+    longest = String("")
+    for i in range(len(s)):
+        left, right = i, i
+        find_longest(left, right, s, longest)
+        left, right = i, i + 1
+        find_longest(left, right, s, longest)
+    return longest
+
+
+fn find_longest(mut left: Int, mut right: Int, s: String, mut longest: String):
+    while 0 <= left and right < len(s) and s[left] == s[right]:
+        if right - left + 1 > len(longest):
+            longest = s[left : right + 1]
+        left -= 1
+        right += 1
+
+
+from testing import assert_true
+
+
+fn main() raises:
+    var s: String = "babad"
+    var expected: String = "bab"
+    result = longest_palindrome(s)
+    assert_true(result == expected, "Assertion failed")
+
+    s = "cbbd"
+    expected = "bb"
+    result = longest_palindrome(s)
+    assert_true(result == expected, "Assertion failed")
+
+    s = "racecar"
+    expected = "racecar"
+    result = longest_palindrome(s)
+    assert_true(result == expected, "Assertion failed")
+
+```
+
+
+[Source](https://github.com/ratulb/mojo_programming/blob/main/codes/longest_palidromic_substr.mojo)