update

hard/297.serialize-and-deserialize-binary-tree.java

@@ -12,7 +12,7 @@ import java.util.LinkedList;
 public class Codec {
 
     // Encodes a tree to a single string.
-    public String serialize(TreeNode root) {
+    public String serialize(TreeNode root) { // Level-order traversal
         StringBuilder sb = new StringBuilder();
 		LinkedList<TreeNode> list = new LinkedList<TreeNode>();
 		list.add(root);
@@ -28,14 +28,14 @@ public class Codec {
 			list.add(curr.left);
 			list.add(curr.right);
 		}
-		sb.deleteCharAt(sb.length()-1);
+		sb.deleteCharAt(sb.length()-1); // Remove the last ','
 		return sb.toString();
     }
 
     // Decodes your encoded data to tree.
     public TreeNode deserialize(String data) {
 		String[] strs = data.split(",");
-		if (strs[0].equals("null")) return null;
+		if (strs[0].equals("null")) return null; // Root is null
 		TreeNode root = new TreeNode(Integer.parseInt(strs[0]));
 		LinkedList<TreeNode> list = new LinkedList<TreeNode>();
 		list.add(root);