add binary tree level order traversal solution

Author: Tessa1217

binary-tree-level-order-traversal/Tessa1217.java

@@ -0,0 +1,87 @@
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+
+/**
+ * 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 {
+
+    // DFS 풀이
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> result = new ArrayList<>();
+        dfs(root, 0, result);
+        return result;
+    }
+
+    private void dfs(TreeNode node, int depth, List<List<Integer>> result) {
+        if (node == null) {
+            return;
+        }
+
+        // 깊이만큼의 리스트가 없으면 새 리스트 추가하기
+        if (depth == result.size()) {
+            result.add(new ArrayList<>());
+        }
+
+        result.get(depth).add(node.val);
+
+        dfs(node.left, depth + 1, result);
+        dfs(node.right, depth + 1, result);
+
+
+    }
+
+    // BFS로 풀이
+    // O(n)
+//    public List<List<Integer>> levelOrder(TreeNode root) {
+//
+//        List<List<Integer>> result = new ArrayList<>();
+//
+//        if (root == null) {
+//            return result;
+//        }
+//
+//        Queue<TreeNode> queue = new LinkedList<>();
+//        queue.offer(root);
+//
+//        while (!queue.isEmpty()) {
+//            int nodeCnt = queue.size();
+//            List<Integer> currentLevelNodes = new ArrayList<>();
+//
+//            for (int i = 0; i < nodeCnt; i++) {
+//                TreeNode current = queue.poll();
+//                currentLevelNodes.add(current.val);
+//
+//                if (current.left != null) {
+//                    queue.offer(current.left);
+//                }
+//
+//                if (current.right != null) {
+//                    queue.offer(current.right);
+//                }
+//
+//            }
+//
+//            result.add(currentLevelNodes);
+//
+//        }
+//
+//        return result;
+//
+//    }
+}
+