1392. Longest Happy Prefix

Problem

A string is called a happy prefix if is a non-empty prefix which is also a suffix (excluding itself).

Given a string s, return **the *longest happy prefix* of** s. Return an empty string "" if no such prefix exists.

  Example 1:

Input: s = "level"
Output: "l"
Explanation: s contains 4 prefix excluding itself ("l", "le", "lev", "leve"), and suffix ("l", "el", "vel", "evel"). The largest prefix which is also suffix is given by "l".

Example 2:

Input: s = "ababab"
Output: "abab"
Explanation: "abab" is the largest prefix which is also suffix. They can overlap in the original string.

  Constraints:

• 1 <= s.length <= 10^5

• s contains only lowercase English letters.

Solution

class Solution {
    public String longestPrefix(String s) {
        int times = 2;
        long prefixHash = 0;
        long suffixHash = 0;
        long multiplier = 1;
        long len = 0;
        // use some large prime as a modulo to avoid overflow errors, e.g. 10 ^ 9 + 7.
        long mod = 1000000007;
        for (int i = 0; i < s.length() - 1; i++) {
            prefixHash = (prefixHash * times + s.charAt(i)) % mod;
            suffixHash = (multiplier * s.charAt(s.length() - i - 1) + suffixHash) % mod;
            if (prefixHash == suffixHash) {
                len = (long) i + 1;
            }
            multiplier = multiplier * times % mod;
        }
        return s.substring(0, (int) len);
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).