백준 1325. 효율적인 해킹 :: 돼지개발자

출저 : https://www.acmicpc.net/problem/1325

"아직도 모르겠는 문제...ㅎㅎ"

나는 처음에 해당 문제를 DFS DP를 이용해서 풀었다. 아래와 같이... 근데 시간초과가 나더라... ㅎㅎ 생각의 전환이 필요했다.

<DFS DP 로 푼 소스>

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.StringTokenizer;
 
public class Main {
 
    static int n,m;
    static ArrayList<Integer>[] list;
    static int[] dp;
    static boolean[] visited;
    static int max = Integer.MIN_VALUE;
    
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());
         
        n = Integer.parseInt(st.nextToken());
        m = Integer.parseInt(st.nextToken());
        
        visited = new boolean[n+1];
        list = new ArrayList[n+1];
        dp = new int[n+1];
        
        for (int i = 0; i <= n; i++) {
            list[i] = new ArrayList<>();
        }
        
        for (int i = 0; i < m; i++) {
            st = new StringTokenizer(br.readLine());
            int a = Integer.parseInt(st.nextToken());
            int b = Integer.parseInt(st.nextToken());
            list[b].add(a);
        }
        for (int i = 1; i <= n; i++) {
            visited[i] = true;
            int res = solve(i);
            visited[i] = false;
            max = Math.max(max, res);
        }
        
        StringBuilder sb = new StringBuilder();
 
        for (int i = 1; i <= n; i++) {
            if(dp[i] == max) 
                sb.append(i+" ");
        }
        System.out.println(sb.toString());
    }
    
    static int solve(int cur) {
        if(dp[cur] != 0) return dp[cur];
        
        dp[cur] = 1;
        
        for( int next : list[cur]) {
            if(visited[next])
                continue;
            
            visited[next] = true;
            dp[cur] += solve(next);
            visited[next] = false;
        }
        
        return dp[cur];
    }
    
}

<맞은 소스>

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;
 
public class Main {
 
    static int n,m;
    static ArrayList<Integer>[] list;
    static int[] dp;
    static boolean[] visited;
    static int max = Integer.MIN_VALUE;
    
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());
         
        n = Integer.parseInt(st.nextToken());
        m = Integer.parseInt(st.nextToken());
        
        list = new ArrayList[n+1];
        dp = new int[n+1];
        
        for (int i = 0; i <= n; i++) {
            list[i] = new ArrayList<>();
        }
        
        for (int i = 0; i < m; i++) {
            st = new StringTokenizer(br.readLine());
            int a = Integer.parseInt(st.nextToken());
            int b = Integer.parseInt(st.nextToken());
            list[a].add(b);
        }
        for (int i = 1; i <= n; i++) {
            visited = new boolean[n+1];
            solve(i);
        }
        
        for (int i = 1; i <= n; i++) {
            max = Math.max(max, dp[i]);
        }
        
        StringBuilder sb = new StringBuilder();
 
        for (int i = 1; i <= n; i++) {
            if(dp[i] == max) 
                sb.append(i+" ");
        }
        System.out.println(sb.toString());
    }
    
    static void solve(int cur) {
        visited[cur] = true;
        
        for( int next : list[cur]) {
            if(visited[next])
                continue;
            dp[next]++;
            solve(next);
        }
    }
    
}