#289 added longestpalindromic substring (#291)

* added longestpalindromic substring

* added longestpalindromic substring

---------

Co-authored-by: albinsabu2023 <[email protected]>

C++/LongestPalindromicSubstring.cpp

@@ -0,0 +1,71 @@
+#include <iostream>
+#include <string>
+using namespace std;
+/**
+ * @brief
+ * author @albinsabu2023
+ * time complexity O(n*n)
+ * space complexity O(n)
+ *
+ * approach:expand around index
+ * two cases
+ * odd length palindrome (single center index)
+ * even length palindrome (two center index)
+ *
+ */
+class Solution
+{
+public:
+    // Function to expand around the center and find the palindrome
+    string expand(string s, int left, int right)
+    {
+        int n = s.length();
+        while (left >= 0 && right < n && s[left] == s[right])
+        {
+            left--;
+            right++;
+        }
+        return s.substr(left + 1, right - left - 1); // return the palindromic substring
+    }
+
+    // Function to find the longest palindromic substring
+    string longestPalindrome(string s)
+    {
+        if (s.length() == 1)
+            return s; // if the string has only one character, it's the longest palindrome
+        int n = s.length();
+        string ans = ""; // variable to store the longest palindrome
+
+        for (int i = 0; i < n - 1; i++)
+        {
+            // Check for odd-length palindromes (centered at one character)
+            string oddlength = expand(s, i, i);
+            if (oddlength.length() > ans.length())
+            {
+                ans = oddlength;
+            }
+
+            // Check for even-length palindromes (centered between two characters)
+            string evenlength = expand(s, i, i + 1);
+            if (evenlength.length() > ans.length())
+            {
+                ans = evenlength;
+            }
+        }
+
+        return ans;
+    }
+};
+
+int main()
+{
+    Solution sol;
+    string s;
+    cout << "Enter the string: ";
+    cin >> s;
+
+    string result = sol.longestPalindrome(s);
+    cout << "Longest palindromic substring is: " << result << endl;
+
+    return 0;
+}