# 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.
```
Example 3:

```Input: s = "leetcodeleet"
Output: "leet"
```
Example 4:

```Input: s = "a"
Output: ""
```

Constraints:

• 1 <= s.length <= 10^5
• s contains only lowercase English letters

Solution:

KMP

```class Solution {
public String longestPrefix(String s) {
int n = s.length(), fallback = -1;
int[] table = new int[n];
table[0] = -1;
for (int i = 1; i < n; i ++) {
while (fallback > -1 && s.charAt(fallback + 1) != s.charAt(i)) {
fallback = table[fallback];
}
if (s.charAt(fallback + 1) == s.charAt(i)) {
fallback ++;
}
table[i] = fallback;
}
return s.substring(0, fallback + 1);
}
}```