214. Shortest Palindrome

214. Shortest Palindrome (Hard)

Given a string s, you can 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.

 

Example 1:

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

Example 2:

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

 

Constraints:

• 0 <= s.length <= 5 * 104
• s consists of lowercase English letters only.

Related Topics:
String

Similar Questions:

• Longest Palindromic Substring (Medium)
• Implement strStr() (Easy)
• Palindrome Pairs (Hard)

Solution 1. Rolling Hash

Find the longest palindrome starting at index 0.

// OJ: https://leetcode.com/problems/shortest-palindrome/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    string shortestPalindrome(string s) {
        unsigned d = 16777619, h = 0, rh = 0, p = 1, maxLen = 0;
        for (int i = 0; i < s.size(); ++i) {
            h = h * d + s[i] - 'a';
            rh += (s[i] - 'a') * p;
            p *= d;
            if (h == rh) maxLen = i + 1;
        }
        auto sub = s.substr(maxLen);
        reverse(begin(sub), end(sub));
        return sub + s;
    }
};