https://leetcode-cn.com/problems/serialize-and-deserialize-binary-tree/submissions/
开始看题没审题，以为要像案例那样使用层序遍历，导致代码超时
改用先序遍历之后，代码简洁不少，也不超时

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

private void doS(TreeNode root, StringBuilder sb) {
    if (root == null) {
        sb.append("n,");
        return;
    }
    sb.append(root.val);
    sb.append(",");
    doS(root.left, sb);
    doS(root.right, sb);
}

// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
    String[] datas = data.split(",");
    if ("n".equals(datas[0])) {
        return null;
    }
    int len = datas.length;
    TreeNode root = doS(datas, len - 1);
    return root;
}

int k = 0;

private TreeNode doS(String[] datas, int len) {
    if (k >= len) {
        return null;
    }
    if ("n".equals(datas[k])) {
        k++;
        return null;
    }
    TreeNode node = new TreeNode(Integer.parseInt(datas[k]));
    k++;
    node.left = doS(datas, len);
    node.right = doS(datas, len);
    return node;
}

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