297. Serialize and Deserialize Binary Tree

题目：

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.

链接： http://leetcode.com/problems/serialize-and-deserialize-binary-tree/

题解：

序列化Binary Tree。 这里因为给定的Tree的val是Integer，所以我们可以用一个字符型的常量当做delimiter，比如','。然后我们可以使用两种方法， pre-order traversal，或者level-order traversal。两种方法的时间复杂度和空间复杂度都一样。下面是pre-order traversal的：

Time Complexity - O(n)， Space Compleixty - O(n)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    private final String delimiter = ",";
    private final String emptyNode = "#";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        return sb.toString();
    }

    private void serialize(TreeNode root, StringBuilder sb) {
        if(root == null) {
            sb.append(emptyNode).append(delimiter);
        } else {
            sb.append(root.val).append(delimiter);
            serialize(root.left, sb);
            serialize(root.right, sb);
        }
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Deque<String> nodes = new LinkedList<>();
        nodes.addAll(Arrays.asList(data.split(delimiter)));
        return deserialize(nodes);
    }

    private TreeNode deserialize(Deque<String> nodes) {
        String nodeVal = nodes.pollFirst();
        if(nodeVal.equals(emptyNode)) {
            return null;
        } else {
            TreeNode node = new TreeNode(Integer.parseInt(nodeVal));
            node.left = deserialize(nodes);
            node.right = deserialize(nodes);
            return node;
        }
    }
}

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

二刷:

总的来说就是用in-order traversal来遍历。  Serialize的时候用一个StringBuilder保存。 Deserialize的时候先根据delimiter把元素都split到一个String[]数组里，然后可以把元素放入queue中，接下来递归使用in-order traversal方法来重建树。重建时每次从queue的前部poll()就可以了。

Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    private final String delimiter = ",";
    private final String nullNode = "#";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        //sb.setLength(sb.length() - 1);
        return sb.toString();
    }

    private void serialize(TreeNode root, StringBuilder sb) {
        if (root == null) {
            sb.append(nullNode).append(delimiter);
        } else {
            sb.append(root.val).append(delimiter);
            serialize(root.left, sb);
            serialize(root.right, sb);
        }
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null) return null;
        String[] strs = data.split(delimiter);
        Queue<String> q = new LinkedList<>();
        Collections.addAll(q, strs);
        return deserialize(q);
    }

    private TreeNode deserialize(Queue<String> q) {
        if (q.isEmpty()) return null;
        String val = q.poll();
        if (val.equals(nullNode)) return null;
        TreeNode node = new TreeNode(Integer.valueOf(val));
        node.left = deserialize(q);
        node.right = deserialize(q);
        return node;
    }
}

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

三刷:

还是使用了in-order traversal。要注意delimiter假如使用'|'会报错，也不知道为什么。

Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    private final String nullNode = "#";
    private final String delimiter = ",";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        return sb.toString();
    }

    private void serialize(TreeNode root, StringBuilder sb) {
        if (root == null) {
            sb.append(nullNode).append(delimiter);
            return;
        }
        sb.append(root.val).append(delimiter);
        serialize(root.left, sb);
        serialize(root.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> q = new LinkedList<>();
        Collections.addAll(q, data.split(delimiter));
        return deserialize(q);
    }

    private TreeNode deserialize(Queue<String> q) {
        if (q.isEmpty()) return null;
        String s = q.poll();
        TreeNode root = null;
        if (!s.equals(nullNode)) {
            root = new TreeNode(Integer.valueOf(s));
            root.left = deserialize(q);
            root.right = deserialize(q);
        }
        return root;
    }
}

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

Reference:

http://articles.leetcode.com/2010/09/serializationdeserialization-of-binary.html

http://articles.leetcode.com/2010/09/saving-binary-search-tree-to-file.html

https://leetcode.com/discuss/66147/recursive-preorder-python-and-c-o-n

https://leetcode.com/discuss/66117/easy-to-understand-java-solution

https://leetcode.com/discuss/66698/java-bfs-based-accepted-solution-using-queue

https://leetcode.com/discuss/66181/easy-to-understand-java-solution

https://leetcode.com/discuss/69390/simple-solution-%23preorder-traversal-%23recursive-%23simple-logic

https://leetcode.com/discuss/66077/c-accepted-o-n-easy-solution

https://leetcode.com/discuss/66397/java-stack-based-solution-using-json

https://leetcode.com/discuss/66076/java-solution-with-queue

https://leetcode.com/discuss/66625/a-level-order-traversal-solution

https://leetcode.com/discuss/66141/a-48-ms-rough-c-solution-used-queue