Add a Java problem of LeetCode Problem

Java/5-Longest_Palindromic_Substring.java

@@ -0,0 +1,63 @@
+public class LongestPalindromicSubstring {
+
+    public String longestPalindrome(String s) {
+        // Update the string to put hash "#" at the beginning, end and in between each character
+        String updatedString = getUpdatedString(s);
+        // Length of the array that will store the window of palindromic substring
+        int length = 2 * s.length() + 1;
+        // Array to store the length of each palindrome centered at each element
+        int[] p = new int[length];
+        // Current center of the longest palindromic string
+        int c = 0;
+        // Right boundary of the longest palindromic string
+        int r = 0;
+        // Maximum length of the substring
+        int maxLength = 0;
+        // Position index
+        int position = -1;
+        for (int i = 0; i < length; i++) {
+            // Mirror of the current index
+            int mirror = 2 * c - i;
+            // Check if the mirror is outside the left boundary of current longest palindrome
+            if (i < r) {
+                p[i] = Math.min(r - i, p[mirror]);
+            }
+            // Indices of the characters to be compared
+            int a = i + (1 + p[i]);
+            int b = i - (1 + p[i]);
+            // Expand the window
+            while (a < length && b >= 0 && updatedString.charAt(a) == updatedString.charAt(b)) {
+                p[i]++;
+                a++;
+                b--;
+            }
+            // If the expanded palindrome is expanding beyond the right boundary of
+            // the current longest palindrome, then update c and r
+            if (i + p[i] > r) {
+                c = i;
+                r = i + p[i];
+            }
+            if (maxLength < p[i]) {
+                maxLength = p[i];
+                position = i;
+            }
+        }
+        int offset = p[position];
+        StringBuilder result = new StringBuilder();
+        for (int i = position - offset + 1; i <= position + offset - 1; i++) {
+            if (updatedString.charAt(i) != '#') {
+                result.append(updatedString.charAt(i));
+            }
+        }
+        return result.toString();
+    }
+
+    private String getUpdatedString(String s) {
+        StringBuilder sb = new StringBuilder();
+        for (int i = 0; i < s.length(); i++) {
+            sb.append("#").append(s.charAt(i));
+        }
+        sb.append("#");
+        return sb.toString();
+    }
+}

Java/README.md

@@ -3,5 +3,6 @@
 ### NOTE : Include the code in the list in order of leetcode problem statement number
 
 1 : Two Sum<br>
+5 : Longest Palindromic Substring<br>
 7 : Reverse Integer<br>
 9 : Palindrome Number<br>