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.

题目含义：实现二叉树的序列化和反序列化算法

思路：按照先序顺序序列化，将每个值用逗号分隔，构造序列化结果

          反序列化时候按照逗号分割出数组，由于是先序，所以数组第一个位置的元素即为根节点，从后面找到第一个大于根节点值的节点mid，即为右子树节点，所以从left到mid-1即为左子树，从mid到right即为右子树

public class Codec {

// Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb  = new StringBuilder();
        serializeDFS(root, sb); //preorder root - left - right
        return sb.toString();
    }
    
    public void serializeDFS(TreeNode root, StringBuilder sb){
        if(root == null) return;
        sb.append(root.val).append(",");
        serializeDFS(root.left, sb);
        serializeDFS(root.right, sb);
    }
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == null || data.length() == 0) return null;
        String arr[] = data.split(",");
        return deserializeDFS(arr, 0, arr.length-1);
    }
    
    public TreeNode deserializeDFS(String[] array, int left, int right){
        if(left > right || left >= array.length) return null;
        TreeNode root = new TreeNode(Integer.parseInt(array[left]));
        int mid = getNextGreater(array, left, right);
        root.left = deserializeDFS(array, left + 1, mid - 1);
        root.right = deserializeDFS(array, mid, right);
        return root;
    }
    
    public int getNextGreater(String[] array, int left, int right){
        int index = 0;
        System.out.println(left);
        int init = Integer.parseInt(array[left]);
        while(left <= right){
            if(init < Integer.parseInt(array[left])) {
                break;
            }
            else left++;
        }
        return left;
    }
}

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