leetcode ShortestPalindrome
z
 2019-04-09 10:36:27   
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public class ShortestPalindrome {
  /**
   * Given a string s, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.
   *
   * Example 1:
   *
   * Input: "aacecaaa"
   * Output: "aaacecaaa"
   * Example 2:
   *
   * Input: "abcd"
   * Output: "dcbabcd"
   */
  public static void main(String[] args){
    String[] test = {"hello", "aaa", "a", "de"};
    for (String s: test){
//      System.out.println(isPalidrome(s));
      System.out.println(shortestPalindrome(s));
    }
  }
  public static String shortestPalindrome(String s) {
    if (isPalidrome(s))
      return s;
    s =  new StringBuffer(s).reverse().toString();
    String ss = s;
    int length = 1;
    while(!isPalidrome(ss)){
      ss = s + new StringBuffer(s.substring(0, length)).reverse().toString();
      length ++;
    }
    return ss;
  }

  public static boolean isPalidrome(String s){
    char[] l = s.toCharArray();
    if (l.length == 1 || (l.length == 2 && l[0] == l[1])){
      return true;
    }
    else if (l.length > 2 && isPalidrome(s.substring(1, s.length()-1)) && l[0] == l[l.length-1]){
      return true;
    }
    else{
      return false;
    }
  }
}