Adding solution to LongestPallindromicSubstring (#106)

* Added Regular Expression matching

* Revert "Added Regular Expression matching"

This reverts commit 4449207.

* adding solution of daily probs

Author: afrinc

LeetCode/Algorithms/Medium/LongestPalindromicSubstring/LongestPalindromicSubstring.java

@@ -0,0 +1,39 @@
+public class LongestPallindromicSubstring {
+
+    public String longestPalindrome(String s) {
+        if (s == null || s.length() < 1) return "";
+        int start = 0;
+        int end = 0;
+
+        for (int i = 0; i < s.length(); i++) {
+            int len1 = expandFromMiddle(s, i, i); // racecar
+            int len2 = expandFromMiddle(s, i, i + 1); // abba
+            int len = Math.max(len1, len2);
+            if (len > end - start) {
+                start = i - ((len - 1) / 2);
+                end = i + (len / 2);
+            }
+        }
+        return s.substring(start, end + 1);
+    }
+
+    // racecar
+    // 0123456
+    // i = 3
+    // len1 = 7
+    // start = 3 - (7-1/2) = 0
+    // end = 3 + 7/2 = 3 + 3 = 6
+
+    public int expandFromMiddle(String s, int left, int right) {
+        if (s == null || left > right)
+            return 0;
+
+        while (left >= 0 && right < s.length() &&
+                s.charAt(left) == s.charAt(right)) {
+            left--;
+            right++;
+        }
+        return right - left - 1; // -1 for that extra right++ increment in while loop
+    }
+}
+