LeetCode-in-Java
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 <= 105scontains only lowercase English letters.
Solution
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);
}
}