Leetcode-449. Serialize and Deserialize BST

前言：为了后续的实习面试，开始疯狂刷题，非常欢迎志同道合的朋友一起交流。因为时间比较紧张，目前的规划是先过一遍，写出能想到的最优算法，第二遍再考虑最优或者较优的方法。如有错误欢迎指正。博主首发优快云，mcf171专栏。

博客链接：mcf171的博客

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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 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.

这个题目可以结合BTS的特点，否则需要保存2n长度的树长度，才知道每一个位置在哪。 Your runtime beats 19.61% of java submissions.

/**
 * 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("");
      Stack<TreeNode> stack = new Stack<TreeNode>();
      stack.push(root);
      while(stack.size() != 0){
          root = stack.pop();
          if(root != null){
            stack.push(root.right);
            stack.push(root.left);
            sb.append(root.val);
            sb.append(",");
          }
      }
      return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        System.out.println(data);
        if(data.length() == 0) return null;
        String[] numbers = data.split(",");
        TreeNode root = new TreeNode(Integer.parseInt(numbers[0]));
        TreeNode iteratorNode = root;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        for(int i = 1; i < numbers.length; i ++){
            TreeNode node = new TreeNode(Integer.parseInt(numbers[i]));
            if(node.val < iteratorNode.val) {iteratorNode.left = node;stack.push(iteratorNode);}
            else {
                while(!stack.empty()){
                    TreeNode parentNode = stack.pop();
                    if(node.val < parentNode.val) {iteratorNode.right = node;stack.push(parentNode);break;}
                    else iteratorNode = parentNode;
                }
                if(stack.empty()) iteratorNode.right = node;

            }
            iteratorNode = node;
        }
        
        return root;
    }
}

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