findCheapestPrice

    public int findCheapestPrice(int n, int[][] flights, int src, int dst, int k) {
        // src-> dest,price
        Map<Integer, Map<Integer, Integer>> prices = new HashMap<>();
        for(int[] flight:flights){
            if(!prices.containsKey(flight[0])) prices.put(flight[0],new HashMap<Integer,Integer>());
            prices.get(flight[0]).put(flight[1],flight[2]);
        }
        PriorityQueue<int[]> pq = new PriorityQueue<>((a,b)->a[0]-b[0]);
        pq.offer(new int[]{0,src,k+1});
        while(!pq.isEmpty()){
            int[] cur = pq.poll();
            int price=cur[0];
            int city = cur[1];
            int stop = cur[2];
            if(city==dst) return price;
            if(stop>0){
                Map<Integer, Integer> adj = prices.getOrDefault(city, new HashMap<>());
                for(int a:adj.keySet()){
                    pq.offer(new int[]{price+adj.get(a),a,stop-1});
                }
            }
        }
        return -1;
    }