297. Serialize and Deserialize Binary Tree

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

For example, you may serialize the following tree

    1
   / \
  2   3
     / \
    4   5

as"[1,2,3,null,null,4,5]", just the same as how LeetCode OJ serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Note:Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

Analysis:

Pre order traversal to serialize, record all NULL for all existing node. The null value will serve as a stop sign.

Complexity:

Time: serialize O(N) deserialize O(N)

Space: O(N)

Code:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {

    private static final String NULL = "X";
    private static final String DEL = ",";
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if(root == null)
        {
            return NULL;
        }
        StringBuilder sb = new StringBuilder();
        buildTreeString(root, sb);
        return sb.toString();
    }

    private void buildTreeString(TreeNode root, StringBuilder sb){
        if(root == null){
            sb.append(NULL);
            sb.append(DEL);
            return;
        }
        sb.append(Integer.valueOf(root.val));
        sb.append(DEL);
        buildTreeString(root.left, sb);
        buildTreeString(root.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data.equals(NULL))
        {
            return null;
        }
        Deque<String> queue = new LinkedList<String>();
        queue.addAll(Arrays.asList(data.split(DEL)));
        return buildTree(queue);
    }

    private TreeNode buildTree(Deque<String> nodesQueue){
        String cur = nodesQueue.poll();
        if(cur.equals(NULL)){
            return null;
        }
        TreeNode root = new TreeNode(Integer.valueOf(cur));
        root.left = buildTree(nodesQueue);
        root.right = buildTree(nodesQueue);
        return root;
    }
}

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