Offer.P37[序列化二叉树]

方法一：BFS

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {

    private static final String EMPTY = "null";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) return "";
        StringBuilder builder = new StringBuilder();
        Deque<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            if (cur == null) {
                builder.append(EMPTY).append(",");
            } else {
                builder.append(cur.val).append(",");
                queue.offer(cur.left);
                queue.offer(cur.right);
            }
        }
        return builder.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.equals("")) return null;
        String[] split = data.split(",");
        Deque<String> nodes = new LinkedList<>(List.of(split));
        TreeNode root = new TreeNode(Integer.parseInt(nodes.poll()));
        Deque<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            String left = nodes.poll();
            String right = nodes.poll();
            if (!left.equals(EMPTY)) {
                cur.left = new TreeNode(Integer.parseInt(left));
                queue.offer(cur.left);
            }
            if (!right.equals(EMPTY)) {
                cur.right = new TreeNode(Integer.parseInt(right));
                queue.offer(cur.right);
            }
        }
        return root;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));

方法二：DFS

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {

    private static final String EMPTY = "null";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) return EMPTY + ",";
        String left = serialize(root.left);
        String right = serialize(root.right);
        return root.val + "," + left + right;
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] nodes = data.split(",");
        Deque<String> queue = new LinkedList<>(List.of(nodes));
        return buildTree(queue);
    }

    public TreeNode buildTree(Deque<String> queue) {
        String cur = queue.poll();
        if (cur.equals(EMPTY)) return null;
        TreeNode node = new TreeNode(Integer.parseInt(cur));
        node.left = buildTree(queue);
        node.right = buildTree(queue);
        return node;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));