214. Shortest Palindrome

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".

先找到字符串左边最长的对称字符串子串，再将对称字符子串右边部分添加到字符串最左边即可。本方法效率较低，但很好理解，程序如下所示：

class Solution {
    public String shortestPalindrome(String s) {
        int len = s.length();
        int maxLen = 1;
        for (int i = 0; i < len; ++ i){
            if (isPalindrome(s, i, i)){
                maxLen = Math.max(maxLen, 2*i + 1);
            }
            if (isPalindrome(s, i, i + 1)){
                maxLen = Math.max(maxLen, 2*i + 2);
            }
        }
        StringBuilder sb = new StringBuilder(s);
        for (int i = maxLen; i < len; ++ i){
            sb.insert(0, s.charAt(i));
        }
        return sb.toString();
    }
    
    public boolean isPalindrome(String s, int i, int j){
        int len = s.length();
        while (i >= 0&&j < len&&s.charAt(i) == s.charAt(j)){
            i --;
            j ++;
        }
        return (i == -1);
    }
}