297. Serialize and Deserialize Binary Tree

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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.

解法1： Preorder + Queue

先用Preorder构造serialization，在deserialize的时候用一个queue存放所有的node的string。
如果是“#”那么直接返回null，否则就知道当前的是一个node，然后分别递归剩下的，得到root.left和root.right的值。

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/**
 * 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) {
        List<String> temp = new ArrayList<>();
        helper(root, temp);
        // join them using ,
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < temp.size() - 1; i++) {
            sb.append(temp.get(i) + ",");
        }
        sb.append(temp.get(temp.size() - 1));
        return sb.toString();
    }
    private void helper(TreeNode root, List<String> sb) {
        if (root == null) {
            sb.add("#");
            return;
        }
        sb.add(Integer.toString(root.val));
        helper(root.left, sb);
        helper(root.right, sb);
    }
    // [1,2,#,#,3,4,#,#,5,#,#]
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null || data.length() == 0) {
            return null;
        }    
        String[] comp = data.split(",");
        Queue<String> queue = new LinkedList<>();
        for (String c : comp) {
            queue.offer(c);
        }
        return deserialize(queue);
    }
    private TreeNode deserialize(Queue<String> queue) {
        if (queue.isEmpty()) {
            return null;
        }
        String rootVal = queue.poll();
        if (rootVal.equals("#")) {
            return null;
        }
        TreeNode root = new TreeNode(Integer.parseInt(rootVal));
        root.left = deserialize(queue);
        root.right = deserialize(queue);
        return root;        
    }
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));