[20260122] BOJ / P5 / 거의 최단 경로 / 김민진

zinnnn37/202601/22 BOJ P5 거의 최단 경로.md

@@ -0,0 +1,143 @@
+```java
+import java.io.*;
+import java.util.*;
+
+public class BJ_5719_거의_최단_경로 {
+
+    private static final BufferedReader  br = new BufferedReader(new InputStreamReader(System.in));
+    private static final BufferedWriter  bw = new BufferedWriter(new OutputStreamWriter(System.out));
+    private static final StringBuilder   sb = new StringBuilder();
+    private static       StringTokenizer st;
+
+    private static int N, M, S, D, shortest;
+    private static int[] dist, distFor, distBack;
+    private static List<Node>[] graphFor, graphBack;
+    private static Queue<Node> pq;
+
+    private static class Node implements Comparable<Node> {
+        int to;
+        int cost;
+
+        public Node(int to, int cost) {
+            this.to = to;
+            this.cost = cost;
+        }
+
+        @Override
+        public int compareTo(Node o) {
+            return Integer.compare(this.cost, o.cost);
+        }
+
+    }
+
+    public static void main(String[] args) throws IOException {
+        while (init()) {
+            sol();
+        }
+        bw.write(sb.toString());
+        bw.flush();
+        bw.close();
+        br.close();
+    }
+
+    private static boolean init() throws IOException {
+        st = new StringTokenizer(br.readLine());
+        N = Integer.parseInt(st.nextToken());
+        M = Integer.parseInt(st.nextToken());
+
+        if (N == 0 && M == 0) return false;
+
+        st = new StringTokenizer(br.readLine());
+        S = Integer.parseInt(st.nextToken());
+        D = Integer.parseInt(st.nextToken());
+
+        graphFor = new List[N];
+        graphBack = new List[N];
+        for (int i = 0; i < N; i++) {
+            graphFor[i] = new ArrayList<>();
+            graphBack[i] = new ArrayList<>();
+        }
+
+        dist = new int[N];
+        distFor = new int[N];
+        distBack = new int[N];
+        Arrays.fill(dist, 987654321);
+        Arrays.fill(distFor, 987654321);
+        Arrays.fill(distBack, 987654321);
+
+        for (int i = 0; i < M; i++) {
+            st = new StringTokenizer(br.readLine());
+            int a = Integer.parseInt(st.nextToken());
+            int b = Integer.parseInt(st.nextToken());
+            int c = Integer.parseInt(st.nextToken());
+
+            graphFor[a].add(new Node(b, c));
+            graphBack[b].add(new Node(a, c));
+        }
+
+        pq = new PriorityQueue<>();
+        return true;
+    }
+
+    private static void sol() {
+        dijkstra(S, D, distFor, graphFor);
+        dijkstra(D, S, distBack, graphBack);
+        shortest = distFor[D];
+
+        findRoute();
+    }
+
+    private static void dijkstra(int start, int end, int[] dist, List<Node>[] graph) {
+        pq.clear();
+        pq.add(new Node(start, 0));
+        dist[start] = 0;
+
+        while (!pq.isEmpty()) {
+            Node cur = pq.poll();
+
+            if (dist[cur.to] < cur.cost) continue;
+
+            if (cur.to == end) break;
+
+            for (Node next : graph[cur.to]) {
+                int nextCost = dist[cur.to] + next.cost;
+
+                if (nextCost < dist[next.to]) {
+                    dist[next.to] = nextCost;
+                    pq.add(new Node(next.to, nextCost));
+                }
+            }
+        }
+    }
+
+    private static void findRoute() {
+        pq.clear();
+        pq.offer(new Node(S, 0));
+        dist[S] = 0;
+
+        while (!pq.isEmpty()) {
+            Node cur = pq.poll();
+
+            if (dist[cur.to] < cur.cost) continue;
+
+            if (cur.to == D) {
+                sb.append(dist[cur.to]).append("\n");
+                return;
+            }
+
+            for (Node next : graphFor[cur.to]) {
+                if (distFor[cur.to] + next.cost + distBack[next.to] == shortest) continue;
+
+                int nextCost = dist[cur.to] + next.cost;
+
+                if (nextCost < dist[next.to]) {
+                    dist[next.to] = nextCost;
+                    pq.offer(new Node(next.to, nextCost));
+                }
+            }
+        }
+        sb.append("-1\n");
+    }
+
+}
+```