while循环内的递归

Question

I am working on a leetcode problem Reconstruct Itinerary , here is a very concise and accepted answer , which is also pasted below. I usually use recursion inside a for loop. I don't understand how exactly the visit works and why it outputs the reversed of the itinerary? Could you please help me digest this code? I never used recursion in while loop before and it may have certain advantages over that in for loop.

class Solution:
    def findItinerary(self, tickets: List[List[str]]) -> List[str]:
        graph=collections.defaultdict(list)
        for edge in tickets:
            graph[edge[0]].append(edge[1])
        for i in graph:
            graph[i].sort(reverse=True)
        ans=[]

        def visit(current):
            while graph[current]:
                visit(graph[current].pop())
            ans.append(current)
            print(ans,current)  # This line added by me

        visit("JFK")
        return ans[::-1]

test case: 
    tickets = [["JFK","KUL"],["JFK","NRT"],["NRT","JFK"]]
output:
    ans[::-1] 
      ['JFK', 'NRT', 'JFK', 'KUL']

Answer 1

Imagine a tree where each node has a variable number of children. It could be 0 or 10,000 or any other non negative integer. The while loop is simply traversing each child node of node current . Observe that

while graph [current] 

is true while the list is nonempty. Each element of the list at current is passed to the next level of recursion at the call site by popping the next element. Specifically

graph[current].pop()

simultaneously removes the element from the list and passes to the next level of recursion.

ans is in reverse order because all nodes are fully explored depth first before the current node is appended. You can see this by moving

ans.append(current)

To before the while loop.