214. Shortest Palindrome

Problem

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".

Solutions

``````public class Solution {
public String shortestPalindrome(String s) {
char[] arr = s.toCharArray();
for (int i = 1; i <= s.length(); i ++) {
if (isPa(arr, 0, s.length() - i) == true) {
return new StringBuilder(s.substring(s.length() - i + 1)).reverse().toString() + s;
}
}
return "";
}
private boolean isPa(char[] arr, int start, int end) {
while (start < end) {
if (arr[start] != arr[end]) {
return false;
}
start ++;
end --;
}
return true;
}
}
``````
``````public class Solution {
public String shortestPalindrome(String s) {
int i = 0, end = s.length() - 1, j = end;
char[] arr = s.toCharArray();
while (i < j) {
if (arr[i] == arr[j]) {
++i; --j;
} else {
i = 0; --end; j = end;
}
}
return new StringBuilder(s.substring(end + 1)).reverse().toString() + s;
}
}
``````