交通枢纽

/*
思路：
1. 使用流收集每个节点的入度和出度。
2. 使用流过滤并查找满足条件的节点。
*/
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class TrafficHubStream {
    public int findHub(int[][] path) {
        Map<Integer, Integer> inDegree = new HashMap<>();
        Map<Integer, Integer> outDegree = new HashMap<>();
        Set<Integer> nodes = Stream.of(path).flatMapToInt(p -> Stream.of(p[0], p[1]).mapToInt(i -> i)).boxed().collect(Collectors.toSet());
        
        Stream.of(path).forEach(p -> {
            int from = p[0];
            int to = p[1];
            outDegree.put(from, outDegree.getOrDefault(from, 0) + 1);
            inDegree.put(to, inDegree.getOrDefault(to, 0) + 1);
        });
        
        int totalNodes = nodes.size();
        return nodes.stream()
                .filter(node -> outDegree.getOrDefault(node, 0) == 0 && inDegree.getOrDefault(node, 0) == totalNodes - 1)
                .findFirst()
                .orElse(-1);
    }

    public static void main(String[] args) {
        TrafficHubStream th = new TrafficHubStream();
        int[][] path1 = {{0,1},{0,3},{1,3},{2,0},{2,3}};
        int[][] path2 = {{0,3},{1,0},{1,3},{2,0},{3,0},{3,2}};
        System.out.println(th.findHub(path1)); // 输出 3
        System.out.println(th.findHub(path2)); // 输出 -1
    }
}