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CourseSchedule.java
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CourseSchedule.java
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class CourseSchedule {
// dfs
public boolean canFinish1(int numCourses, int[][] prerequisites) {
if (numCourses == 0) return false;
List<List<Integer>> graph = buildGraph1(numCourses, prerequisites);
boolean[] visited = new boolean[numCourses];
boolean[] visiting = new boolean[numCourses];
for (int i = 0; i < numCourses; i++) {
if (!dfs(i, graph, visited, visiting)) {
return false;
}
}
return true;
}
private boolean dfs(int node, List<List<Integer>> graph, boolean[] visited, boolean[] visiting) {
if (visited[node]) return true;
if (visiting[node]) return false;
visiting[node] = true;
for (int nei : graph.get(node)) {
if (!dfs(nei, graph, visited, visiting)) {
return false;
}
}
visited[node] = true;
return true;
}
private List<List<Integer>> buildGraph1(int numCourses, int[][] edges) {
List<List<Integer>> graph = new ArrayList<>();
for (int i = 0; i < numCourses; i++) {
graph.add(new ArrayList<>());
}
for (int[] edge : edges) {
graph.get(edge[0]).add(edge[1]);
}
return graph;
}
// bfs
public boolean canFinish2(int numCourses, int[][] prerequisites) {
if (numCourses == 0) return false;
int[] indegree = new int[numCourses];
List<List<Integer>> graph = buildGraph2(numCourses, prerequisites, indegree);
return bfs(numCourses, graph, indegree);
}
private boolean bfs(int nodes, List<List<Integer>> graph, int[] indegree) {
List<Integer> res = new ArrayList<>();
Queue<Integer> queue = new ArrayDeque<>();
for (int i = 0; i < indegree.length; i++) {
if (indegree[i] == 0) {
queue.offer(i);
}
}
while (!queue.isEmpty()) {
int cur = queue.poll();
res.add(cur);
for (int nei : graph.get(cur)) {
indegree[nei]--;
if (indegree[nei] == 0) {
queue.offer(nei);
}
}
}
return res.size() == nodes;
}
private List<List<Integer>> buildGraph2(int nodes, int[][] edges, int[] indegree) {
List<List<Integer>> graph = new ArrayList<>();
for (int i = 0; i < nodes; i++) {
graph.add(new ArrayList<>());
}
for (int[] edge : edges) {
graph.get(edge[1]).add(edge[0]);
indegree[edge[0]]++;
}
return graph;
}
}