image.png
image.png

解法

深度优先遍历解法

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

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) {
            return "X";
        }
        // 先序遍历，进行序列化
        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) {
        List<String> dataList = new LinkedList<String>(Arrays.asList(data.split(",")));   
        return treeNodeDeserialize(dataList); 
    }

    public TreeNode treeNodeDeserialize(List<String> dataList) {
        String item = dataList.get(0);
        dataList.remove(0);
        if (item.equals("X")) {
            return null;
        }
        // 递归先序遍历，构建二叉树
        TreeNode node = new TreeNode(Integer.valueOf(item));
        TreeNode left = treeNodeDeserialize(dataList);
        TreeNode right = treeNodeDeserialize(dataList);
        node.left = left;
        node.right = right;
        return node;
    }
}

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

广度优先遍历解法

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

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) {
            return "X";
        }
        // 先序遍历，进行序列化
        Queue<TreeNode> q = new LinkedList<>();
        List<String> res = new ArrayList<>();
        q.offer(root);
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            if (node != null) {
                res.add(String.valueOf(node.val));
                q.offer(node.left);
                q.offer(node.right);
            } else {
                res.add("X");
            }
        }
        return String.join(",", res);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.equals("X")) {
            return null;
        }
        List<String> dataList = new ArrayList<>(Arrays.asList(data.split(",")));
        // 利用队列，从上往下构造树
        Queue<TreeNode> q = new LinkedList<>();
        TreeNode root = new TreeNode(Integer.valueOf(dataList.get(0)));
        q.offer(root);
        int cur = 1;
        // 广度优先遍历的反过程，先将分配好的左右子节点放到队列中，
        // 用于后续给这些子节点按序分配子节点
        while (cur < dataList.size()) {  
            TreeNode node = q.poll();
            String left = dataList.get(cur);
            String right = dataList.get(cur + 1);
            if (!left.equals("X")) {
                TreeNode leftNode = new TreeNode(Integer.valueOf(left));
                node.left = leftNode;
                q.offer(leftNode);
            }
            if (!right.equals("X")) {
                TreeNode rightNode = new TreeNode(Integer.valueOf(right));
                node.right = rightNode;
                q.offer(rightNode);
            }
            // 每次取两个子节点
            cur += 2;
        }
        return root;
    }
}

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