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 <= 105`
- `s` contains only lowercase English letters.