This is the performance edition of [this kata](https://www.codewars.com/kata/ulam-sequences). If you didn't do it yet, you should begin there.

---

The Ulam sequence U is defined by `u0=u`, `u1=v`, with the general term `u_n` for `n>2` given by the least integer expressible uniquely as the sum of two distinct earlier terms.  
In other words, the next number is always the smallest, unique sum of any two previous terms.

The first 10 terms of the sequence `U(u0=1, u1=2)` are `[1, 2, 3, 4, 6, 8, 11, 13, 16, 18]`.

* Here, the first term after the initial `1, 2` is obviously `3` since `3=1+2`.
* The next term is `4=1+3` (we don't have to worry about `4=2+2` since it is a sum of a single term instead of two distinct terms)
* `5` is not a member of the sequence since it is representable in two ways: `5=1+4=2+3`, but `6=2+4` is a member (since `3+3` isn't a valid calculation).

You'll have to write a code that creates an Ulam Sequence starting with `u0`, `u1` and containing `n` terms.

---

# ___The pitfall...___

While the passing solutions of the first version could have a time complexity of O(N^(3)) (or even O(N^(4))!!), with 20 tests up to 100 terms only, here you'll have to manage generation of sequences up to 1500 terms before time out.

___Inputs:___

* `u0` and `u1`: integers, greater than or equal to 1
* `n`: integer greater than 1, length of the returned list

___Configuration of the tests:___

```if:python
* 6 small fixed tests
* 20 random tests on small sequences (10 to 30 terms)
* 13 large random tests (1500 terms)
```
```if:ruby
* 6 small fixed tests
* 20 random tests on small sequences (10 to 30 terms)
* 16 large random tests (1500 terms)
```
```if:javascript,go
* 6 small fixed tests
* 40 random tests on small sequences (10 to 30 terms)
* 30 large random tests (2450 terms)
```

---

Description Reference: http://mathworld.wolfram.com/UlamSequence.html

```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 = "import time\nfrom heapq import heappush, heappop, heapify\n \n\ndef ulam_sequence(u, v, numTerms):\n    \n    terms = [u, v] # This is the sequence we are producing.\n    sumsDict = dict([(u, 1), (v,1), (u+v, 1)]) # This is a dict of sums so far.  So e.g. 5:2 means there are 2 ways to get 2 terms in the sequence to add to 5.\n    sumsHeap = [u, v, u+v] # Sorted list of those sums.\n    heapify(sumsHeap) \n    deletedDict = dict() # This is a list of sums that have already reached a value of 2 or more.  No point looking at these again.\n\n    # The idea is to go through the sorted list of sums (previous terms in sequence added together)\n    # and find the minimum of those and add that to the sequence.\n    while True:\n        \n        while True:\n            numtoBeAppended = heappop(sumsHeap)\n            if numtoBeAppended > terms[-1] and numtoBeAppended in sumsDict:\n                break\n            \n            \n        terms.append(numtoBeAppended)\n        if len(terms) >= numTerms:\n            break\n        \n        for term in terms[:-1]:\n            entry = term + numtoBeAppended\n            if entry in sumsDict:\n                # If it's already in the heap then there are more than one ways to sum to \n                # this value, so remove it, it will never be part of the sequence\n                del sumsDict[entry]\n                deletedDict[entry] = True\n            else:\n                # If we haven't already marked it as being seen before (and so, deleted) add it to our heap\n                if entry not in deletedDict:\n                    heappush(sumsHeap, entry)\n                    sumsDict[entry] =    1\n            \n                \n\n    return terms"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'ulam_sequence'
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: 6348