LeetCode 449. Serialize and Deserialize BST

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 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.

思路：

• 使用BFS，把所有空节点保存成null，对于非BST应该是唯一做法。BST只需要保存前序遍历就可以。
• 使用DFS，迭代的编码解码。利用BST左边>中间>右边的特性，可以不保存null，在解码时使用lower和upper判断是否是自己的左右节点。（这个递归我写了一下午，还是需要多做题啊）

代码：

/** * 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) {        StringBuffer sb = new StringBuffer();        serializeDFS(root,sb);        return sb.toString();    }    private void serializeDFS(TreeNode root,StringBuffer sb){        if(root==null) return;        sb.append(root.val+" ");        serializeDFS(root.left, sb);        serializeDFS(root.right, sb);    }    // Decodes your encoded data to tree.    public TreeNode deserialize(String data) {        if(data.length() == 0) return null;        String[] strArr = data.split(" ");        List<Integer> list = new ArrayList<>();        list.add(0);        return deserializeDFS(strArr,list,Integer.MIN_VALUE,Integer.MAX_VALUE);    }    private TreeNode deserializeDFS(String[] data,List<Integer> list,int lower,int upper){        //获取位置        int index = list.get(0);        //如果值不在当前范围返回null        int val = Integer.parseInt(data[index]);        if(val<lower||val>upper) return null;        //如果位置在最后直接返回        if(index==data.length-1) return new TreeNode(val);        //新建点,位置+1,可能有左右孩子        TreeNode node = new TreeNode(val);        list.set(0,++index);        // 下一节点在左边        if(Integer.parseInt(data[list.get(0)])<node.val){            node.left = deserializeDFS(data,list,lower,node.val);        }        // 下一节点在右边        if(Integer.parseInt(data[list.get(0)])>node.val){            node.right = deserializeDFS(data,list,node.val,upper);        }        return node;    }}// Your Codec object will be instantiated and called as such:// Codec codec = new Codec();// codec.deserialize(codec.serialize(root));

• 使用队列把BST的前序遍历恢复，小的数放到新队列，大的数放剩在原来的队列里，递归调用。利用了一个新队列，复杂度略高。

// some notes:    //   5    //  3 6    // 2   7    private TreeNode getNode(Queue<Integer> q) { //q: 5,3,2,6,7        if (q.isEmpty()) return null;        TreeNode root = new TreeNode(q.poll());//root (5)        Queue<Integer> samllerQueue = new LinkedList<>();        while (!q.isEmpty() && q.peek() < root.val) {            samllerQueue.offer(q.poll());        }        //smallerQueue : 3,2   storing elements smaller than 5 (root)        root.left = getNode(samllerQueue);        //q: 6,7   storing elements bigger than 5 (root)        root.right = getNode(q);        return root;    }