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) {
char[] c = s.toCharArray();
int n = c.length;
int[] a = new int[n];
int max = 0;
int i = 1;
while (i < n) {
if (c[max] == c[i]) {
max++;
a[i] = max;
i++;
} else {
if (max > 0) {
max = a[max - 1];
} else {
a[i] = 0;
i++;
}
}
}
return s.substring(0, a[n - 1]);
}
}