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Java-based LeetCode algorithm problem solutions, regularly updated

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package g1301_1400.s1392_longest_happy_prefix;

// #Hard #String #Hash_Function #String_Matching #Rolling_Hash
// #2022_03_17_Time_39_ms_(28.37%)_Space_42.6_MB_(94.23%)

/**
 * 1392 - Longest Happy Prefix\.
 *
 * Hard
 *
 * 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 <= 10⁵
 * *   `s` contains only lowercase English letters.
**/
public 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);
    }
}