LeetCode/java/shortest-palindrome.java

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();
  }
}