tree level order traversal

input.txt

@@ -1,4 +1 @@
-3
-1 3 8
-2 3 4
-1 2 5
+3,9,20,null,null,15,7

src/com/dsa/trees/LevelOrderTraversal.java

@@ -0,0 +1,58 @@
+package com.dsa.trees;
+
+import java.io.BufferedReader;
+import java.io.FileReader;
+import java.io.IOException;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+
+public class LevelOrderTraversal {
+    public static void main(String[] args) {
+        try (BufferedReader reader = new BufferedReader(new FileReader("input.txt"))) {
+            String[] elements = reader.readLine().trim().split(",");
+            Queue<TreeNode> queue = new LinkedList<>();
+            TreeNode root = new TreeNode(Integer.parseInt(elements[0]));
+            queue.offer(root);
+            int index = 1;
+            while (!queue.isEmpty() && index < elements.length) {
+                TreeNode node = queue.poll();
+                if (!elements[index].equalsIgnoreCase("null")) {
+                    node.left = new TreeNode(Integer.parseInt(elements[index]));
+                    queue.offer(node.left);
+                }
+                index++;
+                if (!elements[index].equalsIgnoreCase("null")) {
+                    node.right = new TreeNode(Integer.parseInt(elements[index]));
+                    queue.offer(node.right);
+                }
+                index++;
+            }
+            List<List<Integer>> list = levelOrder(root);
+            for (List<Integer> level : list) {
+                System.out.println(level);
+            }
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    public static List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> res = new LinkedList<>();
+        if (root == null) return res;
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            List<Integer> list = new LinkedList<>();
+            int size = queue.size();
+            for (int i = 0; i < size; i++) {
+                TreeNode node = queue.poll();
+                list.add(node.val);
+                if (node.left != null) queue.offer(node.left);
+                if (node.right != null) queue.offer(node.right);
+            }
+            res.add(list);
+        }
+        return res;
+    }
+}

src/com/dsa/trees/TreeNode.java

@@ -0,0 +1,11 @@
+package com.dsa.trees;
+
+public class TreeNode {
+    int val;
+    TreeNode left;
+    TreeNode right;
+    TreeNode() {}
+    TreeNode(int x) {
+        val = x;
+    }
+}