[20260225] BOJ / P4 / 도시 왕복하기 1 / 이강현

lkhyun/202602/25 P4 도시 왕복하기 1.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 = new ArrayList<>();
+    static int ans = 0;
+
+    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[N + 1];
+        for (int i = 0; i <= N; i++) {
+            adjList[i] = new ArrayList<>();
+        }
+
+        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].add(edges.size());
+            edges.add(new Edge(to, 1));
+            adjList[to].add(edges.size());
+            edges.add(new Edge(from, 0));
+        }
+
+        while (findPath()) ans++;
+        System.out.println(ans);
+    }
+
+    public static boolean findPath() {
+        int[] prev = new int[N+1];
+        Arrays.fill(prev, -1);
+        ArrayDeque<Integer> q = new ArrayDeque<>();
+        q.offer(1);
+        prev[1] = 0;
+
+        while(!q.isEmpty()){
+            int cur = q.poll();
+            if(cur == 2) break;
+            for(int i : adjList[cur]){
+                int next = edges.get(i).to;
+                if(prev[next] == -1 && edges.get(i).weight > 0){
+                    prev[next] = i;
+                    q.offer(next);
+                }
+            }
+        }
+
+        if(prev[2] == -1) return false;
+
+        int cur = 2;
+        while(cur != 1){
+            int idx = prev[cur];
+            edges.get(idx).weight--;
+            edges.get(idx ^ 1).weight++;
+            cur = edges.get(idx ^ 1).to;
+        }
+        return true;
+    }
+}
+```