add day 28

28-CourseSchedule.java

@@ -0,0 +1,48 @@
+class Solution {
+    HashMap<Integer, GraphNode> graph = new HashMap<>();
+
+    public boolean canFinish(int n, int[][] preq) {
+        if (preq.length == 0) return true;
+        int totalCourses = preq.length;
+        for (int[] edge : preq) {
+            GraphNode prev = getOrCreate(edge[0]);
+            GraphNode next = getOrCreate(edge[1]);
+            prev.nodes.add(edge[1]);
+            next.inCount += 1;
+        }
+        ArrayList<Integer> noPreqCourses = new ArrayList<>();
+        for (Map.Entry<Integer, GraphNode> entry : graph.entrySet()) {
+            GraphNode node = entry.getValue();
+            if (node.inCount == 0)
+                noPreqCourses.add(entry.getKey());
+        }
+
+        int coursesDone = 0;
+        while (noPreqCourses.size() > 0) {
+            int course = noPreqCourses.get(noPreqCourses.size() - 1);
+            noPreqCourses.remove(noPreqCourses.size() - 1);
+            for (int nextCourse : graph.get(course).nodes) {
+                GraphNode child = graph.get(nextCourse);
+                child.inCount -= 1;
+                coursesDone++;
+                if (child.inCount == 0)
+                    noPreqCourses.add(nextCourse);
+            }
+        }
+        if (totalCourses == coursesDone) return true;
+        return false;
+
+    }
+
+    public GraphNode getOrCreate(int node) {
+        if (!graph.containsKey(node)) {
+            graph.put(node, new GraphNode());
+        }
+        return graph.get(node);
+    }
+}
+
+class GraphNode {
+    int inCount;
+    ArrayList<Integer> nodes = new ArrayList<Integer>();
+}