feat: binary-tree-level-order-traversal

Author: minji-go

binary-tree-level-order-traversal/minji-go.java

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+/**
+ * <a href="https://leetcode.com/problems/binary-tree-level-order-traversal/">week14-2. binary-tree-level-order-traversal</a>
+ * <li>Description: Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level)</li>
+ * <li>Topics: Tree, Breadth-First Search, Binary Tree</li>
+ * <li>Time Complexity: O(N), Runtime 1ms       </li>
+ * <li>Space Complexity: O(N), Memory 45.3MB    </li>
+ */
+
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        if (root == null) {
+            return Collections.emptyList();
+        }
+
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+
+        List<List<Integer>> results = new ArrayList<>();
+        while (!queue.isEmpty()) {
+            List<Integer> result = new ArrayList<>();
+
+            int size = queue.size();
+            for (int i = 0; i < size; i++) {
+                TreeNode node = queue.poll();
+                result.add(node.val);
+                if (node.left != null) queue.offer(node.left);
+                if (node.right != null) queue.offer(node.right);
+            }
+            results.add(result);
+        }
+
+        return results;
+    }
+}