Add Longest Palindromic Substring in Java (#5184)

Author: apurvavats

archive/j/java/LongestPalindromicSubstring.java

@@ -0,0 +1,58 @@
+public class LongestPalindromicSubstring {
+
+    private static final String usage_msg = "Usage: please provide a string that contains at least one palindrome\n";
+
+    private static int expandAroundCenter(String s, int left, int right) {
+        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
+            left--;
+            right++;
+        }
+        return right - left - 1;
+    }
+
+    private static boolean longestPalindromicSubstring(String s, StringBuilder result) {
+        if (s == null || s.isEmpty()) {
+            result.setLength(0);
+            return false;
+        }
+
+        int start = 0, maxLength = 0;
+        int len = s.length();
+
+        for (int i = 0; i < len; i++) {
+            int len1 = expandAroundCenter(s, i, i);
+            int len2 = expandAroundCenter(s, i, i + 1);
+            int currLen = Math.max(len1, len2);
+
+            if (currLen > maxLength) {
+                start = i - (currLen - 1) / 2;
+                maxLength = currLen;
+            }
+        }
+
+        if (maxLength > 1) {
+            result.setLength(0);
+            result.append(s.substring(start, start + maxLength));
+            return true;
+        }
+
+        result.setLength(0);
+        return false;
+    }
+
+    public static void main(String[] args) {
+        if (args.length != 1) {
+            System.out.print(usage_msg);
+            System.exit(1);
+        }
+
+        String input = args[0];
+        StringBuilder result = new StringBuilder();
+
+        if (longestPalindromicSubstring(input, result) && result.length() > 0) {
+            System.out.println(result);
+        } else {
+            System.out.print(usage_msg);
+        }
+    }
+}