Add longest palindromic substring (#2379)

Strings/LongestPalindromicSubstring.java

@@ -0,0 +1,44 @@
+// Longest Palindromic Substring
+import java.util.Scanner;;
+
+
+class LongestPalindromicSubstring {
+    public static void main(String[] args) {
+        Solution s = new Solution();
+        String str = "";
+        Scanner sc = new Scanner(System.in);
+        System.out.print("Enter the string: ");
+        str = sc.nextLine();
+        System.out.println("Longest substring is : "+s.longestPalindrome(str));
+    }
+}
+
+class Solution {
+    public String longestPalindrome(String s) {
+        if (s == null || s.length() == 0) {
+            return "";
+        }
+        int n = s.length();
+        String maxStr = "";
+        for (int i = 0; i < n; ++i) {
+            for (int j = i; j < n; ++j) {
+                if (isValid(s, i, j) == true) {
+                    if (j - i + 1 > maxStr.length()) { // update maxStr
+                        maxStr = s.substring(i, j + 1);
+                    }
+                }
+            }
+        }
+        return maxStr;
+    }
+
+    private boolean isValid(String s, int lo, int hi) {
+        int n = hi - lo + 1;
+        for (int i = 0; i < n / 2; ++i) {
+            if (s.charAt(lo + i) != s.charAt(hi - i)) {
+                return false;
+            }
+        }
+        return true;
+    }
+}