787. Cheapest Flights Within K Stops - LeetCode Solution

class Solution:
    def findCheapestPrice(self, n: int, flights: List[List[int]], src: int, dst: int, K: int) -> int:
        if src == dst: return 0
        
        d, seen = collections.defaultdict(list), collections.defaultdict(lambda: float('inf'))
        for u, v, p in flights:
            d[u] += [(v, p)]
    
        queue = [(src, -1, 0)]
        
        while queue:
            
            position, k, cost = queue.pop(0)
            
            if position == dst or k == K: 
                continue 
            for nei, p in d[position]:
                if cost + p >= seen[nei]:
                    continue
                else:
                    seen[nei] = cost+p
                    queue += [(nei, k+1, cost+p)]
                
        return seen[dst] if seen[dst] < float('inf') else -1