first commit

BinaryTreeLevelOrderTraversal.java

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+// Time complexity: O(n)
+// Space complexity: O(h)
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+
+        List<List<Integer>> result = new ArrayList<>();
+
+        if(root == null) return result;
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.add(root);
+        while(! queue.isEmpty()) {
+                int size = queue.size();
+                List<Integer> list = new ArrayList<>();
+                for(int i=0; i < size; i++) {
+
+                    TreeNode node = queue.poll();
+                    list.add(node.val);
+
+                    if(node.left != null) {
+                        queue.add(node.left);
+                    }
+                    if(node.right != null) {
+                        queue.add(node.right);
+                    }
+                }
+                result.add(list);
+        }
+        return result;
+    }
+}

CourseSchedule.java

@@ -0,0 +1,52 @@
+
+// Time complexity: O(V+E)
+// space complexity: O(V+E)
+class Solution {
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+
+        int[] indegree = new int[numCourses];
+        Map<Integer, List<Integer>> map = new HashMap<>();
+
+        for(int[] prereq: prerequisites) {
+            int ind = prereq[1];
+            int dep =  prereq[0];
+            indegree[dep]++;
+
+            if(map.get(ind)== null) {
+                map.put(ind, new ArrayList<>());    
+            }
+            map.get(ind).add(dep);
+        }
+
+        Queue<Integer> q = new LinkedList<>();
+        int result = 0;    
+        for(int i = 0; i < numCourses; i++) {
+            if(indegree[i] == 0) {
+                q.add(i);
+                result++;
+            }
+        }
+
+        if (q.isEmpty()) return false;
+        if (result == numCourses) return true;
+
+        while(!q.isEmpty()) {
+            int curr = q.poll();
+            List<Integer> dependencies = map.get(curr);
+
+            if(dependencies != null) {
+
+                for(int dependent: dependencies) {
+                    indegree[dependent]--;
+
+                    if(indegree[dependent] == 0) {
+                        q.add(dependent);
+                        result++;
+                        if(result == numCourses) return true;
+                    }
+                }
+            }
+        }
+        return false;
+    }
+}