Solved BFS 1 problems

course-schedule.java

@@ -0,0 +1,52 @@
+//TC: O(V+E)
+//SC: O(n)
+
+class Solution {
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+        HashMap<Integer, List<Integer>> map = new HashMap<>(); // for dependency list
+        int[] indegrees = new int[numCourses]; //how many pre requistes are there
+
+        for (int[] edge : prerequisites) {
+            int dep = edge[0];
+            int in = edge[1];
+            indegrees[dep]++;
+            if (!map.containsKey(in)) {
+                map.put(in, new ArrayList<>());
+            }
+
+            map.get(in).add(dep);
+        }
+
+        Queue<Integer> q = new LinkedList<>();
+        int count = 0;
+
+        for (int i = 0; i < numCourses; i++) {
+            if (indegrees[i] == 0) {
+                q.add(i);
+                count++;
+                if (count == numCourses)
+                    return true;
+            }
+        }
+
+        if (q.isEmpty())
+            return false;
+
+        while (!q.isEmpty()) {
+            int curr = q.poll();
+            List<Integer> babies = map.get(curr);
+            if (babies != null) {
+                for (int baby : babies) {
+                    indegrees[baby]--;
+                    if (indegrees[baby] == 0) {
+                        q.add(baby);
+                        count++;
+                        if (count == numCourses)
+                            return true;
+                    }
+                }
+            }
+        }
+        return false;
+    }
+}

level-order-traversal.java

@@ -0,0 +1,51 @@
+//TC: O(n)
+//SC: O(1)
+
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        //DFS
+        List<List<Integer>> result = new ArrayList<>();
+        if (root == null)
+            return result;
+        dfs(root, 0, result);
+        return result;
+    }
+    private void dfs(TreeNode curr, int level, List<List<Integer>> result){
+        //base
+        if(curr == null) return;
+
+        //logic
+        if(result.size() == level){
+            result.add(new ArrayList<>());
+        }
+        result.get(level).add(curr.val);
+        dfs(curr.left, level+1, result);
+        dfs(curr.right, level+1, result);
+
+    }
+}
+
+//BFS
+/* List<List<Integer>> result = new ArrayList<>();
+if (root == null)
+    return result;
+Queue<TreeNode> q = new LinkedList<>();
+q.add(root);
+while (!q.isEmpty()) {
+    int size = q.size();
+    List<Integer> li = new ArrayList<>();
+    for (int i = 0; i < size; i++) {
+        TreeNode curr = q.poll();
+        li.add(curr.val);
+        if (curr.left != null) {
+            q.add(curr.left);
+        }
+        if (curr.right != null) {
+            q.add(curr.right);
+        }
+    }
+    result.add(li);
+}
+return result;
+}
+} */