def solve(tickets):
    G = collections.defaultdict(lambda :[])
    for sn, en in tickets:
        G[sn].append(en)
    for key in G.keys():
        G[key].sort()
    r = []
    def dfs(edge):
        while nexts := G[edge]:
            next_edge = nexts.pop(0)
            dfs(next_edge)
        r.append(edge)
    dfs('JFK')
    return r[::-1]