Add longestPalindrome function to find palindromes

Implement a solution to find the longest palindromic substring using the expand from center technique.

leetcode/lc5/ExpandCentre.cpp

@@ -0,0 +1,32 @@
+class Solution {
+public:
+        string longestPalindrome(string s) {
+        if (s.length() <= 1) {
+            return s;
+        }
+
+        auto expand_from_center = [&](int left, int right) {
+            while (left >= 0 && right < s.length() && s[left] == s[right]) {
+                left--;
+                right++;
+            }
+            return s.substr(left + 1, right - left - 1);
+        };
+
+        string max_str = s.substr(0, 1);
+
+        for (int i = 0; i < s.length() - 1; i++) {
+            string odd = expand_from_center(i, i);
+            string even = expand_from_center(i, i + 1);
+
+            if (odd.length() > max_str.length()) {
+                max_str = odd;
+            }
+            if (even.length() > max_str.length()) {
+                max_str = even;
+            }
+        }
+
+        return max_str;
+    }
+};