我的代码超过了时间限制。 如何使我的代码更加优化？

Question

My code exceeds the time limit at test case 23. I have used dfs to traverse the connected houses. I did everything I could to optimize from my side. Kattis, problem: where's my internet? https://open.kattis.com/problems/wheresmyinternet

import java.util.Scanner;
import java.util.HashMap;
import java.util.ArrayList;

class Main{
    static boolean[] isVisited;
    static HashMap<Integer, ArrayList<Integer>> hashHouses;

    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);

        int numberOfHouses = scanner.nextInt();
        int numberOfCables = scanner.nextInt();

        hashHouses = new HashMap<>();

        for (int i = 1; i < numberOfHouses + 1; i++){
            hashHouses.put(i, new ArrayList<Integer>());
        }

        for (int i = 0; i < numberOfCables; i++){
            int x = scanner.nextInt();
            int y = scanner.nextInt();

            hashHouses.get(x).add(y);
            hashHouses.get(y).add(x);
        }

        isVisited = new boolean[numberOfHouses + 1];
        isVisited[1] = true;

        dfs(1);

        boolean isConnected = true;
        for (int i = 1; i < numberOfHouses + 1; i++){
            if (!isVisited[i]){
                System.out.println(i);
                isConnected = false;
            }
        }

        if (isConnected) System.out.println("Connected");
    }

    static void dfs(int start) {
        isVisited[start] = true;
        for (Integer i : hashHouses.get(start)) {
            if (!isVisited[i]) {
                dfs(i);
            }
        }
    }
}

Answer 1

The problem is not in your algorithm. It is just that input and output in Java is too slow for this online judge.

The solution is to use more efficient input and output code:

• For input, use a custom Fast Scanner. See this page for several options.
• For output, write the result in a StringBuilder, and then output this StringBuilder with a single System.out .

I was able to pass all the tests on your problem just by applying these two optimisations.