Added O(n2) java solution for leetcode 5

Author: 619frank

leetcode/medium/5. Longest Palindromic Substring/java/Solution.java

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+class Solution {
+
+    public static void main(String[] args) {
+        String s = "abba";
+        System.out.println(longestPalindromeOn2(s));
+    }
+
+    public static String longestPalindromeOn2(String s) {
+        
+        if(s == null || s.length() < 1) return "";
+        
+        int start = 0;
+        int end = 0;
+        
+        for(int i = 0; i < s.length(); i++){
+            int lenOdd = expandFromMiddle(s, i, i);
+            int lenEven = expandFromMiddle(s, i, i+1);
+            int lenMax = Math.max(lenOdd, lenEven);
+            if(lenMax > end - start){
+                start = i - ((lenMax - 1)/ 2);
+                end = i + (lenMax/2);
+            }
+        }
+        
+        return s.substring(start, end+1);
+        
+    }
+    
+    public static 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;
+        
+    }
+}

leetcode/medium/5. Longest Palindromic Substring/question

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+Given a string s, return the longest palindromic substring in s.  
+
+ 
+
+Example 1:  
+
+Input: s = "babad"  
+Output: "bab"  
+Explanation: "aba" is also a valid answer.  
+Example 2:  
+
+Input: s = "cbbd"  
+Output: "bb"  
+ 
+
+Constraints:  
+
+1 <= s.length <= 1000  
+s consist of only digits and English letters.  
+
+Hint:
+There are three ways to solve it
+1. O(n3) -> for each of all the substring see if its palindrome or not
+2. O(n2) -> Expand From middle Approach
+3. O(n) -> Manacher's Algorithm -> Very hard