剑指 Offer 37. 序列化二叉树

输入：root = [1,2,3,null,null,4,5]
输出：[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) {
        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            int len = queue.size();
            for (int i = 0; i < len; i++) {
                TreeNode node = queue.poll();
                // 这里直接追加逗号，这样在split进行分割的时候，就会把末尾的空字符全部去掉
                // 对于示例，得到的字符串是1,2,3,,,4,5,,,,
                // split进行分割后，得到字符串数组["1", "2", "3", "", "", "4", "5"]
                if (node == null) {
                    sb.append(",");
                } else {
                    sb.append(node.val).append(",");
                    queue.offer(node.left);
                    queue.offer(node.right);
                }
            }
        }
        return sb.substring(0, sb.length() - 1).toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<TreeNode> queue = new LinkedList<>();
        String[] vals = data.split(",");
        TreeNode root = (!"".equals(vals[0])) ? (new TreeNode(Integer.parseInt(vals[0]))) : null;
        TreeNode node = root;
        queue.offer(node);
        int index = 1;
        while (!queue.isEmpty() && index < vals.length) {
            int len = queue.size();
            for (int i = 0; i < len && index < vals.length; i++) {
                TreeNode temp = queue.poll();
                if (temp == null) {
                    continue;
                }
     
                String str = vals[index++];
                temp.left = !"".equals(str) ? new TreeNode(Integer.parseInt(str)) : null;
                if (index >= vals.length) {
                    break;
                }
                str = vals[index++];
                temp.right = !"".equals(str) ? new TreeNode(Integer.parseInt(str)) : null;
                queue.offer(temp.left);
                queue.offer(temp.right);
            }
        }

        return root;
    }
}

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