297. Serialize and Deserialize Binary Tree
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.
For example, you may serialize the following tree1 / \ 2 3 / \ 4 5
as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.
Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.
思路
理论上说所有遍历的方法都可以。但是为了使serialize和deserialize的过程都尽量最简单,preorder是不错的选择。serialize的话,dfs比较好写,deserialize的话preorder和bfs比较好写。用“,”作为分隔符,“#”来表示null。例子里serialize之后结果就变成"1,2,3,#,#,4,5"。deserialize的时候用一个queue来保存string。
复杂度
Time: O(N), Space: O(N)
代码
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
// base case
if(root == null) return "";
StringBuilder encoded = new StringBuilder();
encode(root, encoded);
return encoded.substring(1).toString();
}
private void encode(TreeNode root, StringBuilder sb) {
if(root == null) {
sb.append(",#");
return;
}
sb.append(",").append(root.val);
encode(root.left, sb);
encode(root.right, sb);
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
// base case
if(data.length() == 0) return null;
Queue<String> q = new LinkedList(Arrays.asList(data.split(",")));
return decode(q);
}
private TreeNode decode(Queue<String> q) {
if(q.isEmpty()) return null;
String cur = q.poll();
if(cur.equals("#")) return null;
TreeNode root = new TreeNode(Integer.valueOf(cur));
root.left = decode(q);
root.right = decode(q);
return root;
}
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.
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
这道题和之前不同,一般的树变成了BST,而且要求是as compact as possible。还是可以用preorder,还是需要分隔符,但是null就不需要保存了。deserialize部分要变得复杂,left的值总是小于root的值,right的值总是大于root的值,根据这个每次recursion的时候把左边的值都放到另一个queue里面,剩下的就是右边的值。
复杂度
Time: O(N^2), Space: O(N)
代码
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
// base case
if(root == null) return "";
StringBuilder encoded = new StringBuilder();
encode(root, encoded);
return encoded.substring(1).toString();
}
private void encode(TreeNode root, StringBuilder sb) {
if(root == null) return;
sb.append(",").append(root.val);
encode(root.left, sb);
encode(root.right, sb);
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
// base case
if(data.length() == 0) return null;
Queue<Integer> q = new LinkedList();
for(String s : data.split(",")) q.offer(Integer.valueOf(s));
return decode(q);
}
private TreeNode decode(Queue<Integer> q) {
if(q.isEmpty()) return null;
int cur = q.poll();
TreeNode root = new TreeNode(cur);
Queue<Integer> left = new LinkedList();
while(!q.isEmpty() && q.peek() < cur) left.offer(q.poll());
root.left = decode(left);
root.right = decode(q);
return root;
}
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