binary-tree-level-order-traversal solution

Author: Jeongwon Na

binary-tree-level-order-traversal/njngwn.java

@@ -0,0 +1,43 @@
+/**
+ * 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;
+ *     }
+ * }
+ */
+class Solution {
+    // Time Complexity: O(n), n: the number of nodes
+    // Space Complexity: O(w), w: max width of tree
+
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> results = new ArrayList<>();
+        if (root == null) return results;
+
+        Queue<TreeNode> levelQueue = new LinkedList<>();    // use queue considering FIFO
+        levelQueue.offer(root);
+
+        while (!levelQueue.isEmpty()) {
+            ArrayList<Integer> level = new ArrayList<>();   // node values list for each level
+            int size = levelQueue.size();
+
+            for (int i = 0; i < size; ++i) {
+                TreeNode node = levelQueue.poll();
+                level.add(node.val);
+                if(node.left != null) levelQueue.offer(node.left);
+                if(node.right != null) levelQueue.offer(node.right);
+            }
+
+            results.add(level);
+        }
+
+        return results;
+    }
+}