题解 | 24点运算

def choose(cands: list[str]):
    c = len(cands)
    for i in range(c):
        yield cands[i], cands[:i] + cands[i + 1 :]


def calcs(a: int, b: int):
    yield a + b, "+"
    yield a - b, "-"
    yield a * b, "*"
    if b != 0:
        yield a // b, "/"


def dfs(cands: list[str], x=None, logs: list[tuple[str, int]] = []):
    if len(cands) == 0:
        return logs if x == 24 else None
    if x is None:
        for c, cs in choose(cands):
            e = dfs(cs, M[c], logs + [(c, M[c])])
            if e is not None:
                return e
    else:
        for c, cs in choose(cands):
            for r, op in calcs(x, M[c]):
                e = dfs(cs, r, logs + [(op + c, r)])
                if e is not None:
                    return e


M = {
    "3": 3,
    "4": 4,
    "5": 5,
    "6": 6,
    "7": 7,
    "8": 8,
    "9": 9,
    "10": 10,
    "J": 11,
    "Q": 12,
    "K": 13,
    "A": 1,
    "2": 2,
}
s = input().split()
if "joker" in s or "JOKER" in s:
    print("ERROR")
else:
    logs = dfs(s)
    # print(logs)
    print("".join(x[0] for x in logs) if logs is not None else "NONE")