expanding around centre solution

Author: Arjiit

Two Pointers/longestPalindrome.java

@@ -0,0 +1,29 @@
+class Solution {
+// O(N^2) time, O(1) space
+    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 = expandFromCentre(s, i, i); //"racecar" for 'e'
+            int len2 = expandFromCentre(s, i, i+1); //babaabab
+            
+            int max_len = Math.max(len1, len2);
+            if (max_len > end - start) {
+                start = i - ((max_len - 1)/2);
+                end = i + (max_len/2);
+            }
+        }
+        return s.substring(start, end+1);
+    }
+    
+    public int expandFromCentre(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;
+    }
+}