449. 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.

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if(root == null) return "#";
        return root.val + "," + serialize(root.left) + "," + serialize(root.right);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> q = new LinkedList(Arrays.asList(data.split(",")));
        return dehelp(q);
    }
    
    public TreeNode dehelp(Queue<String> q) {
        String s = q.poll();
        if(s.equals("#")) return null;
        
        TreeNode root = new TreeNode(Integer.valueOf(s));
        root.left = dehelp(q);
        root.right = dehelp(q);
        return root;
    }
}

嗯？不就是serialize吗，我直接用binary tree的，可以是可以，就是速度有点慢，肯定要用到bst性质。

和297最重要的区别是as compact as possible，意味着少用点#（表示null）

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        return sb.toString();
    }
    
    public void serialize(TreeNode root, StringBuilder sb) {
        if (root == null) return;
        sb.append(root.val).append(",");
        serialize(root.left, sb);
        serialize(root.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.isEmpty()) return null;
        Queue<String> q = new LinkedList<>(Arrays.asList(data.split(",")));
        return deserialize(q, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }
    
    public TreeNode deserialize(Queue<String> q, int lower, int upper) {
        if (q.isEmpty()) return null;
        String s = q.peek();
        int val = Integer.parseInt(s);
        if (val < lower || val > upper) return null;
        q.poll();
        TreeNode root = new TreeNode(val);
        root.left = deserialize(q, lower, val);
        root.right = deserialize(q, val, upper);
        return root;
    }
}