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Copy pathCourseSchedule.java
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81 lines (69 loc) · 2.45 KB
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class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
List<List<Integer>> graph = new ArrayList<>();
for(int i = 0; i < numCourses; i++) {
graph.add(new ArrayList<>());
}
Map<Integer, Integer> dMap = fillinDegreeAndGraph(prerequisites, graph);
Queue<Integer> queue = new ArrayDeque<>();
for (int i = 0; i < numCourses; i++) {
if (!dMap.containsKey(i)) {
queue.offer(i);
}
}
int cnt = 0;
while (!queue.isEmpty()) {
int n = queue.poll();
cnt++;
for (int x : graph.get(n)) {
dMap.put(x, dMap.get(x) - 1);
if (dMap.get(x) == 0) {
queue.offer(x);
}
}
}
return cnt == numCourses;
}
private Map<Integer, Integer> fillinDegreeAndGraph(int[][] edges, List<List<Integer>> graph) {
Map<Integer, Integer> map = new HashMap<>();
for (int[] x : edges) {
graph.get(x[1]).add(x[0]);
map.put(x[0], map.getOrDefault(x[0], 0) + 1);
}
return map;
}
}
// java 8 solution
class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
if(prerequisites == null || prerequisites.length == 0) return true;
Map<Integer, List<Integer>> graph = new HashMap<>();
Map<Integer, Integer> dMap = new HashMap<>();
fillInGraphAndDegree(prerequisites, graph, dMap);
Queue<Integer> queue = new ArrayDeque<>();
for(int i = 0; i < numCourses; i++) {
if(!dMap.containsKey(i)){
queue.offer(i);
}
}
int count = 0;
while(!queue.isEmpty()) {
Integer t = queue.poll();
for(Integer x : graph.getOrDefault(t, Collections.emptyList())){
Integer degree = dMap.merge(x, -1, Integer::sum);
if(degree == 0) queue.offer(x);
}
count++;
}
return count == numCourses;
}
private void fillInGraphAndDegree(int[][] edges, Map<Integer, List<Integer>> graph, Map<Integer, Integer> dMap) {
for(int[] edge: edges) {
graph.merge(edge[1], new ArrayList<>(Collections.singleton(edge[0])), (x, y) -> {
x.add(edge[0]);
return x;
});
dMap.merge(edge[0], 1, Integer::sum);
}
}
}