297. Serialize and Deserialize Binary Tree.md

297. Serialize and Deserialize Binary Tree

Question

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.

Example:

You may serialize the following tree:

    1
   / \
  2   3
     / \
    4   5

as "[1,2,3,null,null,4,5]"

Thinking:

• Method 1: 使用了额外内存，cheating
  • [1,[2,[],[]],[3,[4,[],[]],[5,[],[]]]]

/**
 * 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) {
        if(root == null) return "[]";
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        sb.append(root.val);
        sb.append(",");
        sb.append(serialize(root.left));
        sb.append(",");
        sb.append(serialize(root.right));
        sb.append("]");
        System.out.println(sb.toString());
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    private int pos = 0;
    public TreeNode deserialize(String data) {
        char[] arr = data.toCharArray();
        if(arr[pos++] != '[') return null;
        if(arr[pos] == ']'){
            pos++;
            return null;
        }
        StringBuilder num = new StringBuilder();
        while(arr[pos] != ','){
            num.append(arr[pos++]);
        }
        pos++;
        TreeNode cur = new TreeNode(Integer.valueOf(num.toString()));
        cur.left = deserialize(data);
        if(arr[pos++] != ',') return null;
        cur.right = deserialize(data);
        if(arr[pos++] != ']') return null;
        return cur;
    }
}

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

Second time

/**
 * 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 split = ",";
    private final String empty = "#";

    // 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 node, StringBuilder sb){
        if(node == null){
            sb.append(empty).append(split);
        }else{
            sb.append(node.val).append(split);
            serialize(node.left, sb);
            serialize(node.right, sb);
        }
    }
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] nodes = data.split(split);
        LinkedList<String> q = new LinkedList<>();
        q.addAll(Arrays.asList(nodes));
        return deserialize(q);
    }
    private TreeNode deserialize(LinkedList<String> q){
        String val = q.poll();
        if(val.equals(empty)){
            return null;
        }else{
            TreeNode node = new TreeNode(Integer.parseInt(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));

Third Time

• Method 1: dfs

  /**
   * Definition for a binary tree node.
   * public class TreeNode {
   *     int val;
   *     TreeNode left;
   *     TreeNode right;
   *     TreeNode(int x) { val = x; }
   * }
   */
  public class Codec {
      private static final String EMPTY = "#";
      private static final String SPLIT = ",";
      // 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 node, StringBuilder sb){
          if(node == null){
              sb.append(EMPTY).append(SPLIT);
          }else{
              sb.append(node.val).append(SPLIT);
              serialize(node.left, sb);
              serialize(node.right, sb);
          }
      }
      // Decodes your encoded data to tree.
      public TreeNode deserialize(String data) {
          String[] tokens = data.split(SPLIT);
          LinkedList<String> q = new LinkedList<>();
          q.addAll(Arrays.asList(tokens));
          return deserialize(q);
      }
      private TreeNode deserialize(LinkedList<String> q){
          String token = q.poll();
          if(token.equals(EMPTY)) return null;
          else{
              TreeNode node = new TreeNode(Integer.parseInt(token));
              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));