class Solution:
    def findItinerary(self, tickets: List[List[str]]) -> List[str]:
      graph = collections.defaultdict(list)
      for t in tickets:
        graph[t[0]].append(t[1])
        graph[t[0]].sort()
      res = []
      def helper(node):
        while graph[node]:
          helper(graph[node].pop(0))
        res.append(node)

      helper('JFK')
      return res[::-1]