实现下载历史记录与去重逻辑

def solve():
    class DownloadTracker:
        def __init__(____): self.history=set()
        def record(____):
            key=(____)
            if key in self.history: return False,'____'
            self.history.add(____); return True,'____'
        def user_downloads(____): return [s for u,s in self.history if u==uid]
    t=DownloadTracker()
    tests=[(____),(____),(____),(____)]
    print(____)
    for uid,sid in tests:
        ok,msg=t.record(____)
        print(____)
    print(____)