[LeetCode]（面試題37）序列化二叉樹

題目

請實現兩個函數，分別用來序列化和反序列化二叉樹。

示例:

你可以將以下二叉樹：

    1
   / \
  2   3
     / \
    4   5

序列化爲 "[1,2,3,null,null,4,5]"

解題思路

詳細思路請參考 297. 二叉樹的序列化與反序列化

代碼

解法一：DFS（先序遍歷）

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 // 解法一：DFS(先序遍歷)
public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder res = serialize(root, new StringBuilder());
        return res.toString();
    }

    private StringBuilder serialize(TreeNode root, StringBuilder str){
        if(root == null){
            str.append("null,");
        }else{
            str.append(root.val);
            str.append(",");
            str = serialize(root.left, str);
            str = serialize(root.right, str);
        }
        return str;
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] strData = data.split(",");
        List<String> list = new LinkedList<String>(Arrays.asList(strData));
        return deserialize(list);
    }

    private TreeNode deserialize(List<String> list){
        if(list.get(0).equals("null")){
            list.remove(0);
            return null;
        }
        TreeNode root = new TreeNode(Integer.valueOf(list.get(0)));
        list.remove(0);
        root.left = deserialize(list);
        root.right = deserialize(list);
        return root;
    }
}

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

解法二：BFS（層序遍歷）

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
// 解法二：BFS(層序遍歷)
public class Codec {

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


    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] strData = data.split(",");
        if(strData[0].equals("null")){
            return null;
        }
        TreeNode root = new TreeNode(Integer.valueOf(strData[0]));
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int i = 1;
        while(!queue.isEmpty()){
            TreeNode cur = queue.poll();
            if(!strData[i].equals("null")){
                cur.left = new TreeNode(Integer.valueOf(strData[i]));
                queue.offer(cur.left);
            }
            i++;
            if(!strData[i].equals("null")){
                cur.right = new TreeNode(Integer.valueOf(strData[i]));
                queue.offer(cur.right);
            }
            i++;
        }
        return root;
    }
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));