05 August 2008
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.
297是普通二叉树，这道题是BST，对于二叉搜索树而言，左子节点 < 根节点 < 右子节点；根据这个性质，用先序遍历序列（根左右）就可以进行重建了，并且不需要存储空指针。 如果是一个普通二叉树，需要前序和中序一起，或者后序和中序两个序列，才能确定唯一的二叉树结构，而对于BST，由于各元素之间有左子节点小于父节点再小于右子节点的性质，因此用一个前序遍历就可以确定出唯一结构，所以要简单些。
当然，也可以用297的一个层序遍历来确定，同297，只是不用用BST的特性来优化了。
Preorder
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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) {
StringBuilder sb = new StringBuilder();
serialize(root, sb);
return sb.toString();
}
private void serialize(TreeNode root, StringBuilder sb) {
if (root == null) {
return;
}
// 先序遍历拼接当前节点
sb.append(root.val).append(","); // 使用分隔符来区别树节点
serialize(root.left, sb);
serialize(root.right, sb);
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
if (data.isEmpty()) {
return null;
}
// 用数组来做二分查找，比用297中用的queue要方便一些
String[] arr = data.split(",");
return deserialize(arr, 0, arr.length - 1); //去掉分隔符后的数组来重构BST
}
private TreeNode deserialize(String[] arr, int low, int high) {
if (low > high) {
return null;
}
TreeNode root = new TreeNode(Integer.parseInt(arr[low])); // 建立父节点
int index = high + 1; // 初始化，右子树为空的情况
for (int i = low + 1; i <= high; i++) {
if (Integer.parseInt(arr[i]) > root.val) {
index = i;
break;
}
}
// 递归构建子树，二分查找优化
root.left = deserialize(arr, low + 1, index - 1);
root.right = deserialize(arr, index, high);
return root;
}
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));