[level order and can finish]

Solution.java

@@ -0,0 +1,74 @@
+class Solution {
+    // Time Complexity: O(V + E)
+    // Space Complexity: O(V + E)
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+        int indegrees [] = new int[numCourses];
+        Map<Integer, List<Integer>> map = new HashMap<>();
+        Queue<Integer> q = new LinkedList<>();
+
+        // have a key where key is the course that is prerequisite
+        for (int  [] pair : prerequisites ) {
+            indegrees[pair[0]]++;
+            List<Integer> list = map.getOrDefault(pair[1], new ArrayList<>());
+            list.add(pair[0]);
+            map.put(pair[1], list);
+        }
+        // if course doesnt have prerequisite add into the queue and keep numCourses left to ge through
+        for (int i = 0; i < indegrees.length; i++) {
+            if (indegrees[i] == 0) {
+                q.add(i);
+                numCourses--;
+            }
+        }
+
+        int size = q.size();
+        int temp = 0;
+
+        // q contains courses that can be completed so far
+        while (size != 0) {
+            size = q.size();
+            if (size == 0) return numCourses == 0;
+            temp = q.poll();
+            List<Integer> li = map.getOrDefault(temp, new ArrayList());
+            for (int i = 0; i < li.size(); i++) {
+                // if a course no longer have prerequisites add to q and decrements the num of courses left
+                if (--indegrees[li.get(i)] == 0) {
+                    q.add(li.get(i));
+                    numCourses--;
+                }
+            }
+
+            if(li.size() != 0) map.remove(temp);
+        }
+        // if there are no courses that cant be completed return true
+        return numCourses == 0;
+    }
+
+
+
+    ///  level order with recursive
+
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> ans = new ArrayList<>();
+        //keep track of the level
+        helper(root, ans, 0);
+
+        return ans;
+
+    }
+
+    public void helper(TreeNode rt, List<List<Integer>> ans, int level) {
+        if (rt == null) return;
+
+        if (ans.size() == level) {
+            ans.add(new ArrayList<>());
+        }
+        ans.get(level).add(rt.val);
+
+
+        helper(rt.left, ans, level + 1);
+        helper(rt.right, ans, level + 1);
+
+
+    }
+}