package LeetCode_1392
/**
* 1392. Longest Happy Prefix
* https://leetcode.com/problems/longest-happy-prefix/
*
* 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.
* */
class Solution {
/*
* solution: KMP, find a prefix which also is a suffix,
* Time complexity:O(n), Space complexity:O(n)
* */
fun longestPrefix(s: String): String {
if (s == null || s.isEmpty()) {
return ""
}
val lpsTable = createLPSTable(s)
val last = lpsTable[lpsTable.lastIndex]
return if (last == 0) "" else s.substring(0, last)
}
/**
* Create LPS table or pi table
* */
private fun createLPSTable(string: String): IntArray {
var i = 1//i for scan string
var j = 0//j from scan patten
val m = string.length
val lps = IntArray(m)
lps[0] = 0//lps[0] is always 0
//loop for i = 1 to m-1
while (i < m) {
if (string[i] == string[j]) {
//if two char are same, move i,j
lps[i] = j + 1
i++
j++
} else {
/*
* if not the same, 2 situation:
* 1. if (j==0), then cannot move j to left, move i to right, and update lps[i]
* 2. if (j>0), move j to table[j-1]
* */
if (j == 0) {
lps[i] = j
i++
} else {
j = lps[j - 1]
}
}
}
return lps
}
}
