LeetCode in Kotlin
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
class Solution {
fun longestPrefix(s: String): String {
val times = 2
var prefixHash: Long = 0
var suffixHash: Long = 0
var multiplier: Long = 1
var len: Long = 0
// use some large prime as a modulo to avoid overflow errors, e.g. 10 ^ 9 + 7.
val mod: Long = 1000000007
for (i in 0 until s.length - 1) {
prefixHash = (prefixHash * times + s[i].code.toLong()) % mod
suffixHash = (multiplier * s[s.length - i - 1].code.toLong() + suffixHash) % mod
if (prefixHash == suffixHash) {
len = i.toLong() + 1
}
multiplier = multiplier * times % mod
}
return s.substring(0, len.toInt())
}
}