Shortest Palindrome

Hard

You are given a string s. You may convert it to a palindrome by adding characters in front of it. Return the shortest palindrome you can find by performing this transformation.

Example 1

Input:  s = "aacecaaa"
Output: "aaacecaaa"

Example 2

Input:  s = "abcd"
Output: "dcbabcd"

Example 3

Input:  s = ""
Output: ""

Constraints

• 0 <= s.length <= 16
• s consists of lowercase English letters only.