序列化二叉树

题目：请实现两个函数，分别用来序列化和反序列化二叉树

思路：层顺遍历二叉树 并保存所有的空节点

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
　　　// 层序遍历最后结果 [1,2,3,null,null,4,5,null,null,null,null]

 // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) return "[]";
        StringBuilder sb = new StringBuilder("[");
        LinkedList<TreeNode> q = new LinkedList<>();
        q.add(root);
        while(q.size() > 0 ) {
            TreeNode t = q.removeFirst();
            if (t == null) {
                sb.append("null").append(",");
            }else {
                sb.append(t.val).append(",");
                q.add(t.left);
                q.add(t.right);
            }
        }
　　　　　　// 移除最后一个多余的 ,
　　　　　sb.deleteCharAt(res.length() - 1);
        return sb.append("]").toString();
    }



    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data.equals("[]")) return null;
        String[] vals = data.substring(1, data.length() - 1).split(",");
        TreeNode root = new TreeNode(Integer.parseInt(vals[0]));
        Queue<TreeNode> queue = new LinkedList() {{ add(root); }};
        int i = 1;
        while(!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if(!vals[i].equals("null")) {
                node.left = new TreeNode(Integer.parseInt(vals[i]));
                queue.add(node.left);
            }
            i++;
            if(!vals[i].equals("null")) {
                node.right = new TreeNode(Integer.parseInt(vals[i]));
                queue.add(node.right);
            }
            i++;
        }
        return root;
    }