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.

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = "class Solution:\n    def longestPrefix(self, s: str) -> str:\n        '''\n        res keeps track of the index of the end of the prefix, used to output the happy prefix\n        l stores the hash key for prefix\n        r tracks the hash key for suffix\n        '''\n        res, l, r = 0, 0, 0\n        mod = 10**9 + 5 # used to ensure that the hash value doesn't overflow\n        '''\n        now start from the beginning and end of the string\n        - note you shouldn't search teh whole string because the longest prefix/suffix is the string itself\n        '''\n        for i in range(len(s)-1):\n            # hash the prefix/suffix in o(1); 128 can be substituted with any value; it just happens to be the length of ASCII\n            '''\n            for a given string 'elkmmmelk', prefix elk will be hashed in ascending order.\n            e: 'e': hash[e] = ord(e)\n            l: 'el': hash[el] = hash(e)*128 + ord(l)\n            k: 'elk': hash[elk] = hash(el)*128 + ord(k)\n            which translates to (l * 128 + ord(s[i]))\n\n            suffix elk will be hashed in descending order\n            k: 'k': hash[k] = ord(k)\n            l: 'lk': hash[lk] = ord(k) + ord(l) * 128\n            e: 'elk': hash[elk] = ord(k) + ord(l) * 128 + ord(e) * 128^2\n            which translates to r + pow(128, i, mod)\n            '''\n            l = (l * 128 + ord(s[i])) % mod\n            r = (r + pow(128, i, mod) * ord(s[-i-1])) % mod\n            if l == r: # if both hash values match, update res\n                res = i + 1\n        return s[:res]"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'longestPrefix'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 5550