[20260226] BOJ / P3 / 도시 왕복하기 2 / 이강현

Author: lkhyun

lkhyun/202602/26 BOJ P3 도시 왕복하기 2.md

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+ ```java
+import java.util.*;
+import java.io.*;
+
+public class Main {
+    static class Edge {
+        int to, weight;
+        public Edge(int to, int weight) {
+            this.to = to;
+            this.weight = weight;
+        }
+    }
+    static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+    static StringTokenizer st;
+    static int N, P;
+    static List<Integer>[] adjList;
+    static List<Edge> edges;
+    static final int MAX = Integer.MAX_VALUE / 2;
+
+    public static void main(String[] args) throws Exception {
+        st = new StringTokenizer(br.readLine());
+        N = Integer.parseInt(st.nextToken());
+        P = Integer.parseInt(st.nextToken());
+
+        adjList = new List[2 * N + 1];
+        for (int i = 0; i <= 2 * N; i++) adjList[i] = new ArrayList<>();
+        edges = new ArrayList<>();
+
+        for (int i = 1; i <= N; i++) {
+            int cap = (i == 1 || i == 2) ? MAX : 1;
+            adjList[i].add(edges.size());
+            edges.add(new Edge(i + N, cap));
+            adjList[i + N].add(edges.size());
+            edges.add(new Edge(i, 0));
+        }
+
+        for (int i = 0; i < P; i++) {
+            st = new StringTokenizer(br.readLine());
+            int from = Integer.parseInt(st.nextToken());
+            int to = Integer.parseInt(st.nextToken());
+
+            adjList[from + N].add(edges.size());
+            edges.add(new Edge(to, MAX));
+            adjList[to].add(edges.size());
+            edges.add(new Edge(from + N, 0));
+
+            adjList[to + N].add(edges.size());
+            edges.add(new Edge(from, MAX));
+            adjList[from].add(edges.size());
+            edges.add(new Edge(to + N, 0));
+        }
+
+        int ans = 0;
+        while (findPath()) ans++;
+        System.out.println(ans);
+    }
+
+    public static boolean findPath() {
+        int[] prev = new int[2 * N + 1];
+        Arrays.fill(prev, -1);
+        prev[1] = -2;
+        ArrayDeque<Integer> q = new ArrayDeque<>();
+        q.offer(1);
+
+        while (!q.isEmpty()) {
+            int cur = q.poll();
+            for (int i : adjList[cur]) {
+                int to = edges.get(i).to;
+                if (prev[to] == -1 && edges.get(i).weight > 0) {
+                    prev[to] = i;
+                    if (to == 2) {
+                        int node = 2;
+                        while (node != 1) {
+                            int idx = prev[node];
+                            edges.get(idx).weight--;
+                            edges.get(idx ^ 1).weight++;
+                            node = edges.get(idx ^ 1).to;
+                        }
+                        return true;
+                    }
+                    q.offer(to);
+                }
+            }
+        }
+        return false;
+    }
+}
+```