天题系列： Shortest Palindrome

public String shortestPalindrome(String s) {
if (s == null || s.length() <= 1)
return s;

String result = null;

int len = s.length();
int mid = len / 2;

for (int i = mid; i >= 1; i--) {
if (s.charAt(i) == s.charAt(i - 1)) {
if ((result = scanFromCenter(s, i - 1, i)) != null)
return result;
} else {
if ((result = scanFromCenter(s, i - 1, i - 1)) != null)
return result;
}
}

return result;
}

private String scanFromCenter(String s, int l, int r) {
int i = 1;

//scan from center to both sides
for (; l - i >= 0; i++) {
if (s.charAt(l - i) != s.charAt(r + i))
break;
}

//if not end at the beginning of s, return null
if (l - i >= 0)
return null;

StringBuilder sb = new StringBuilder(s.substring(r + i));
sb.reverse();

return sb.append(s).toString();
}