BFS-1 Done

Leetcode_102.java

@@ -0,0 +1,101 @@
+//way1
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+ //Place root node in queue
+ //Rootnode acts as parent
+ //Travel through queue based on it's size and add them to list
+ //Once travelled till size, add list to ans and continue with new size and so on
+ //ie., travelling parent - it's children - it's grandchildren so on
+ //TC: O(n); SC:O(n)
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> ans=new ArrayList<>();
+        if(root==null){
+            return ans;
+        }
+
+        Queue<TreeNode> queue=new LinkedList<>();
+        queue.add(root);
+        while(!(queue.isEmpty())){
+            int size=queue.size();
+            List<Integer> curr=new ArrayList<>();
+            for(int i=0;i<size;i++){
+                TreeNode currNode=queue.poll();
+                curr.add(currNode.val);
+
+                if(currNode.left!=null){
+                    queue.add(currNode.left);
+                }
+
+                if(currNode.right!=null){
+                    queue.add(currNode.right);
+                }
+
+            }
+
+            ans.add(curr);
+        }
+
+        return ans;
+
+
+    }
+}
+
+//way2
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+ //DFS
+ //Maintain ans as List of List --> reference
+ //Check level at every node
+ //If that level is already present in ans, add it to the subList of ans at level. Else, create new List, add element and add that list to ans
+ //TC: O(n); SC:O(h)
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> ans=new ArrayList<>();
+        helper(root,0,ans);
+        return ans;
+    }
+
+    private void helper(TreeNode root, int level, List<List<Integer>> ans){
+        if(root==null){
+            return;
+        }
+
+
+        if(level==ans.size()){
+            ans.add(new ArrayList<>());
+        }
+
+        ans.get(level).add(root.val);
+
+        helper(root.left,level+1,ans);
+        helper(root.right,level+1,ans);
+    }
+}

Leetcode_207.java

@@ -0,0 +1,63 @@
+//Topological sort
+//Travel through prereq
+//Maintain indegree array; Maintain map with key as indep node and value as list of it's dependents
+//Once the indegree array and map is formed, check number of independent nodes from indegree array
+//If all are independent - we can complete all courses
+//Else, add independent nodes to queue
+//Process one by one. Update indegree array every time. 
+//If all nodes of indegree array becomes 0, no cycle and courses can be completed
+//TC: O(V+E)
+//SC: O(V+E)
+
+class Solution {
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+        int n=numCourses;
+        int[] indegree=new int[n];
+        Map<Integer,List<Integer>> map=new HashMap<>();
+
+        for(int[] req:prerequisites){
+            int dep=req[0];
+            int indep=req[1];
+
+            indegree[dep]+=1;
+
+            if(!(map.containsKey(indep))){
+                map.put(indep,new ArrayList<>());
+            }
+            map.get(indep).add(dep);
+        }
+
+        int count=0;
+        Queue<Integer> queue=new LinkedList<>();
+        for(int i=0;i<n;i++){
+            if(indegree[i]==0){
+                queue.add(i);
+                count+=1;
+            }
+        }
+
+        if(count==n) return true;
+
+        while(!(queue.isEmpty())){
+            int currVal=queue.poll();
+            List<Integer> curr=map.get(currVal);
+            if(curr!=null){
+                for(int c:curr){
+                    indegree[c]-=1;
+                    if(indegree[c]==0){
+                        queue.add(c);
+                        count+=1;
+
+                        if(count==n) return true;
+                    }
+                }
+            }
+        }
+
+        if(count==n) return true;
+        return false;
+
+
+
+    }
+}