java: add «214. Shortest Palindrome»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/shortest-palindrome.java

@@ -0,0 +1,38 @@
+class Solution {
+  private String convert(String s) {
+    var reversed = new StringBuilder(s).reverse();
+
+    var combined = new StringBuilder(s);
+    combined.append('#');
+    combined.append(reversed);
+
+    return combined.toString();
+  }
+
+  private int[] makePrefixTable(String s) {
+    var result = new int[s.length()];
+
+    var length = 0;
+    for (int i = 1; i < s.length(); ++i) {
+      while (length > 0 && s.charAt(i) != s.charAt(length)) {
+        length = result[length - 1];
+      }
+
+      if (s.charAt(i) == s.charAt(length)) {
+        ++length;
+      }
+
+      result[i] = length;
+    }
+
+    return result;
+  }
+
+  public String shortestPalindrome(String s) {
+    var prefix = makePrefixTable(convert(s));
+    var length = prefix[prefix.length - 1];
+
+    var suffix = new StringBuilder(s.substring(length)).reverse();
+    return suffix.append(s).toString();
+  }
+}