SerializeDeserializeBinaryTree.java

import java.util.ArrayList;
import java.util.List;

public class SerializeDeserializeBinaryTree {
    
    public String serialize(TreeNode root) {
        List<String> res = new ArrayList<>();
        dfsSerialize(root, res);
        return String.join(",", res);
    }

    private void dfsSerialize(TreeNode node, List<String> res) {
        if (node == null) {
            res.add("N");
            return;
        }
        res.add(String.valueOf(node.val));
        dfsSerialize(node.left, res);
        dfsSerialize(node.right, res);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] vals = data.split(",");
        int[] i = {0};
        return dfsDeserialize(vals, i);
    }

    private TreeNode dfsDeserialize(String[] vals, int[] i) {
        if (vals[i[0]].equals("N")) {
            i[0]++;
            return null;
        }
        TreeNode node = new TreeNode(Integer.parseInt(vals[i[0]]));
        i[0]++;
        node.left = dfsDeserialize(vals, i);
        node.right = dfsDeserialize(vals, i);
        return node;
    }
}