37-序列化二叉树

请实现两个函数，分别用来序列化和反序列化二叉树。

示例:

你可以将以下二叉树：

    1
   / \
  2   3
     / \
    4   5
序列化为 "[1,2,3,null,null,4,5]"

/**
 * 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 "null";
        StringBuilder sb = new StringBuilder();
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            TreeNode node = queue.poll();
            if (node != null) {
                sb.append(String.valueOf(node.val)).append(",");
                queue.offer(node.left);
                queue.offer(node.right);
            } else {
                sb.append("null").append(",");
            }
        }
        sb.setLength(sb.length() - 1);
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null || data.equals("null")) return null;
        String[] s = data.split(",");
        TreeNode head = new TreeNode(Integer.valueOf(s[0]));
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(head);
        int i = 1;
        while(!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if (i < s.length && !s[i].equals( "null")) {
                node.left = new TreeNode(Integer.valueOf(s[i]));
                queue.offer(node.left);
            }
            i++;
            if (i < s.length && !s[i].equals( "null")) {
                node.right = new TreeNode(Integer.valueOf(s[i]));
                queue.offer(node.right);
            }
            i++;
        }
        return head;
    }
}

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