Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Your algorithm should run in O(n) complexity.

Example:


Input: [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

```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 longestConsecutive(self, nums):\n         """\n         :type nums: List[int]\n         :rtype: int\n         """\n         \n         if len(nums) == 0:\n             return 0\n         \n         seen = set()\n         heads = set()\n         tails = set()\n         forward = dict()\n         reverse = dict()\n         \n         for n in nums:\n             \n             if n in seen:\n                 continue\n             else:\n                 seen.add(n)\n             \n             if n-1 not in tails and n+1 not in heads:\n                 heads.add(n)\n                 tails.add(n)\n                 forward[n] = n\n                 reverse[n] = n\n             \n             if n-1 not in tails and n+1 in heads:\n                 heads.remove(n+1)\n                 heads.add(n)\n                 tail = forward[n+1]\n                 del forward[n+1]\n                 forward[n] = tail\n                 reverse[tail] = n\n             \n             if n-1 in tails and n+1 not in heads:\n                 tails.remove(n-1)\n                 tails.add(n)\n                 head = reverse[n-1]\n                 del reverse[n-1]\n                 reverse[n] = head\n                 forward[head] = n\n             \n             if n-1 in tails and n+1 in heads:\n                 head = reverse[n-1]\n                 tail = forward[n+1]\n                 heads.remove(n+1)\n                 tails.remove(n-1)\n                 del forward[n+1]\n                 del reverse[n-1]\n                 forward[head] = tail\n                 reverse[tail] = head\n         \n         \n         diffs = [v - k + 1 for (k,v) in list(forward.items())]\n         maxRun = max(diffs)\n         \n         return maxRun\n                 \n                 \n                 \n             \n'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'longestConsecutive'
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: 8642