Shortest Palindrome

1. Title

Shortest Palindrome

2.   Http address

https://leetcode.com/problems/shortest-palindrome/

3. The question

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.

For example:

Given "aacecaaa", return "aaacecaaa".

Given "abcd", return "dcbabcd".

4. My code (AC)

 //Accepted
      public  String shortestPalindrome0(String s) {
          if( s == null || s.length() <= 1 || this.isPalindrom(s))
          {
              return s;
          }
          int len = s.length();
          int mid = len /2;
          for(int i = mid; i >= 0; i--)
          {
              StringBuilder sb = new StringBuilder("");
              if(checkIsPalindrom(s,i,sb))
              {
                 // int dis = (len-i) - (i);
                  //System.out.println("outer" + sb.toString());
                  int index = Integer.parseInt(sb.toString());
                  
                  StringBuilder strB = new StringBuilder(s.substring(index));
                  return strB.reverse().toString() + s;
              }
          }
          return "error";
      }
      // 
    private boolean checkIsPalindrom(String s, int mid, StringBuilder sb) {
        // TODO Auto-generated method stub
        int len = s.length();
        int i = mid - 1;
        int j = mid;
        if( mid < 1){
            sb.append(1);
            return true;
        }
        
        
        while( j < len && i >= 0)
        {
            if(s.charAt(j) != s.charAt(i))
            {
                break;
            }
            j++;
            i--;
        }
        
        if( j < len && i >= 0)
        {
            i = mid - 1;
            j = mid + 1;
            if( j >= len)
            {
                return false;
            }
            while( j < len && i >= 0)
            {
                if(s.charAt(j) != s.charAt(i))
                {
                    return false;
                }
                j++;
                i--;
            }
        }
        
        sb.append(j);
        return true;
    }

    public   boolean isPalindrom(String s){
        if( s == null || s.length() <= 1)
            return true;
        int len = s.length();
        int i = 0;
        int j = len -1;
        while( i < j)
        {
            if( s.charAt(i) != s.charAt(j)){
                return false;
            }
            i++;
            j--;
        }
        return true;
    }