[20260227] BOJ / P3 / 도시 왕복하기 2 / 권혁준

khj20006/202602/27 BOJ P3 도시 왕복하기 2.md

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+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+
+    static class IOManager {
+        static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        static BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
+        static StringTokenizer st = new StringTokenizer("");
+
+        private IOManager(){}
+
+        static String nextLine() throws Exception {
+            String line = br.readLine();
+            st = new StringTokenizer(line);
+            return line;
+        }
+
+        static String nextToken() throws Exception {
+            while (!st.hasMoreTokens())
+                nextLine();
+            return st.nextToken();
+        }
+
+        static int nextInt() throws Exception {
+            return Integer.parseInt(nextToken());
+        }
+
+        static long nextLong() throws Exception {
+            return Long.parseLong(nextToken());
+        }
+
+        static double nextDouble() throws Exception {
+            return Double.parseDouble(nextToken());
+        }
+
+        static void write(String content) throws Exception {
+            bw.write(content);
+        }
+
+        public static void close() throws Exception {
+            bw.flush();
+            bw.close();
+            br.close();
+        }
+    }
+
+    //
+
+    static final int INF = (int)1e9 + 7;
+
+    static int N, P;
+    static int ans = 0;
+    static int[][] flow, capacity;
+    static int[] level;
+
+    public static void main(String[] args) throws Exception {
+        N = IOManager.nextInt();
+        P = IOManager.nextInt();
+        flow = new int[2*N+1][2*N+1];
+        capacity = new int[2*N+1][2*N+1];
+        for(int i=3;i<=N;i++) {
+            capacity[i][i+N] = 1;
+        }
+        capacity[1][1+N] = capacity[2][2+N] = INF;
+
+        for(int i=0;i<P;i++) {
+            int a = IOManager.nextInt(), b = IOManager.nextInt();
+            capacity[a+N][b] = 1;
+            capacity[b+N][a] = 1;
+        }
+
+        while(bfs(1, 2)) {
+            while(dfs(1, INF, 2) > 0);
+        }
+        IOManager.write(ans + "\n");
+
+        IOManager.close();
+    }
+
+    public static boolean bfs(int source, int sink) {
+        level = new int[2*N+1];
+        Queue<Integer> queue = new ArrayDeque<>();
+        BitSet visited = new BitSet(2*N+1);
+        queue.add(source);
+        visited.set(source);
+        while(!queue.isEmpty()) {
+            int cur = queue.poll();
+            for(int nxt=1;nxt<=2*N;nxt++) {
+                if(!visited.get(nxt) && capacity[cur][nxt] - flow[cur][nxt] > 0) {
+                    visited.set(nxt);
+                    queue.add(nxt);
+                    level[nxt] = level[cur] + 1;
+                }
+            }
+        }
+        return visited.get(sink);
+    }
+
+    public static int dfs(int cur, int amount, int sink) {
+        if(cur == sink) {
+            ans++;
+            return amount;
+        }
+        for(int nxt=1;nxt<=2*N;nxt++) {
+            int residual = capacity[cur][nxt] - flow[cur][nxt];
+            if(residual > 0 && level[nxt] == level[cur] + 1) {
+                int min = Math.min(amount, residual);
+                int real = dfs(nxt, min, sink);
+                if(real > 0) {
+                    flow[cur][nxt] += real;
+                    flow[nxt][cur] -= real;
+                    return real;
+                }
+            }
+        }
+        return 0;
+    }
+}
+```