Leetcode: Serialize and Deserialize BST

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

The encoded string should be as compact as possible.

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

So the first question is: what is the difference between this and #297?

This here is BST, however, in #297, it's BT. "The encoded string should be as compact as possible" here. The special property of binary search trees compared to general binary trees allows a more compact encoding. So while the solutions to problem #297 still work here as well, they're not as good as they should be.

For general BT, to reconstruct the tree, we need the information of both in-order and pre-order, or in-order and post-order. We've practised that already. 

However, as a BST, just the information of pre-order or post-order is sufficient to rebuild the tree.

The encoded string is also most compact, since we do not need to keep tract of information of 'Null nodes'.

Example:   4

　　　　 2       6

         1   3   5    7

The pre-order encoding is: "4213657". It is easy to tell "4" is root, "213" is left tree, "657" is right tree. We can use a Queue to implement this, very convenient.

/**
 * 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 "";
        StringBuilder res = new StringBuilder();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode node = root;
        while (!stack.isEmpty() || node!=null) {
            if (node != null) {
                res.append(node.val).append(" ");
                stack.push(node);
                node = node.left;
            }
            else {
                node = stack.pop().right;
            }
        }
        return res.toString().trim();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data==null || data.length()==0) return null;
        String[] nodes = data.split(" ");
        Queue<Integer> queue = new LinkedList<>();
        for (String node : nodes) {
            queue.offer(Integer.valueOf(node));
        }
        return buildTree(queue);
    }
    
    public TreeNode buildTree(Queue<Integer> queue) {
        if (queue.isEmpty()) return null;
        TreeNode root = new TreeNode(queue.poll());
        Queue<Integer> leftNodeVals = new LinkedList<>();
        while (!queue.isEmpty() && queue.peek()<=root.val) {
            leftNodeVals.offer(queue.poll());
        }
        root.left = buildTree(leftNodeVals);
        root.right = buildTree(queue);
        return root;
    }
}

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