거의 최단경로

Author: kchung1995

kuyhochung/1. 매주 문제풀이/15주차/5719_거의 최단경로.cpp

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+#include <iostream>
+#include <vector>
+#include <queue>
+#include <cstring>
+using namespace std;
+
+#define MAX_N 500
+#define INF 1e9
+
+int n, m;
+vector<pair<int, int>> edges[MAX_N]; //각 노드 사이의 거리를 담는 배열
+vector<pair<int, int>> trace[MAX_N]; //BFS의 역추적을 위한 배열
+bool visited[MAX_N][MAX_N];
+
+void init () {
+ memset(visited, false, sizeof(visited));
+ memset(trace, 0, sizeof(trace));
+ for (int i = 0; i < MAX_N; i++) {
+ edges[i].clear();
+ }
+}
+
+vector<int> dijkstra(int start, int nodes) {
+ //dijkstra 알고리즘을 위한 거리 배열 - 시작점은 0, 그 외 무한으로 거리 초기화
+ vector<int> distance(nodes, INF);
+ distance[start] = 0;
+
+ priority_queue<pair<int, int>> pq; //-cost, node 번호
+ pq.push({0, start});
+
+ while (!pq.empty()) {
+ int cost = -pq.top().first;
+ int currentNode = pq.top().second;
+ pq.pop();
+
+ if (distance[currentNode] < cost) continue;
+ 
+ for (int i = 0; i < edges[currentNode].size(); i++) {
+ int nextNode = edges[currentNode][i].first;
+ int nextCost = cost + edges[currentNode][i].second;
+ 
+ //삭제된 edge 무시
+ if (edges[currentNode][i].second == -1) continue;
+
+ if (distance[nextNode] > nextCost) {
+ distance[nextNode] = nextCost;
+ pq.push(make_pair(-nextCost, nextNode));
+
+ //trace 갱신
+ trace[nextNode].clear();
+ trace[nextNode].push_back(make_pair(currentNode, nextCost));
+ }
+
+ //최단 거리 찾을 때마다 trace update
+ else if (distance[nextNode] == nextCost)
+ trace[nextNode].push_back(make_pair(currentNode, nextCost));
+ }
+ }
+ return distance;
+}
+
+void BFS(int destination) {
+ //queue를 이용하여 trace에 해당하는 정점들을 모두 지울 준비를 한다
+ queue<int> q;
+ bool visitedNode[MAX_N];
+ for (int i = 0; i < n; i++) visitedNode[i] = false;
+ q.push(destination);
+ visitedNode[destination] = true;
+
+ while (!q.empty()) {
+ int currentNode = q.front();
+ q.pop();
+ 
+ for (int i = 0; i < trace[currentNode].size(); i++) {
+ int nextNode = trace[currentNode][i].first;
+ if (visited[currentNode][nextNode]) continue;
+ 
+ //역순으로 접근하므로 nextNode 기준으로 접근
+ for (int i = 0; i < edges[nextNode].size(); i++) {
+ if (edges[nextNode][i].first == currentNode)
+ edges[nextNode][i].second = -1;
+ }
+ if (!visitedNode[nextNode]) {
+ q.push(nextNode);
+ visitedNode[nextNode] = true;
+ }
+ }
+ }
+}
+
+int main() {
+ ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
+ 
+ while (1) {
+ init();
+
+ cin >> n >> m;
+ if (n == 0 && m == 0) break;
+
+ int start, goal;
+ cin >> start >> goal;
+
+ for (int i = 0; i < m; i++) {
+ int from, to, cost;
+ cin >> from >> to >> cost;
+ edges[from].push_back({to, cost});
+ }
+ 
+ dijkstra(start, n);
+ BFS(goal);
+ vector<int> result = dijkstra(start, n);
+
+ if (result[goal] == INF) cout << -1 << '\n';
+ else cout << result[goal] << '\n';
+ }
+ return 0;
+}