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.
For example, you may serialize the following tree
1
/ \
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.
Thinking:
There are serveral thinkings emerged in my mind when I first saw this question. Firstly, I want to finishi the normal method which is the same as the Leetcode presenting. But it is LTE. Then I searched on the Internet and found my solution’s lack and then solved it.
Solution:
Previous Code:
//normal method
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
String res = "";
if (root == null)
return res;
Queue<TreeNode> q = new LinkedList<TreeNode>();
q.add(root);
res += root.val + ",";
while (!q.isEmpty()){
int num = q.size();
String nullstr = "";
while (num > 0){
TreeNode n = q.poll();
if (n.left != null){
q.add(n.left);
res += nullstr + n.left.val + ",";
}
else{
nullstr += "null,";
}
if (n.right != null){
q.add(n.right);
res += nullstr + n.right.val + ",";
}
else{
nullstr += "null,";
}
num--;
}
}
return res;
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
TreeNode root;
if(data.length() == 0)
return null;
String[] strs = data.split(",");
root = new TreeNode(Integer.parseInt(strs[0]));
bfs(root, strs, 1, true);
bfs(root, strs, 2, false);
return root;
}
private void bfs(TreeNode node, String[] strs, int index, boolean left){
if (index >= strs.length)
return;
if (strs[index].equals("null"))
return;
TreeNode newNode = new TreeNode(Integer.parseInt(strs[index]));
if (left){
node.left = newNode;
}
else{
node.right = newNode;
}
bfs(newNode, strs, index*2+1, true);
bfs(newNode, strs, index*2+2, false);
}
Reference Code(http://blog.csdn.net/ljiabin/article/details/49474445):
public String serialize(TreeNode root) {
StringBuilder sb = new StringBuilder();
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while (!queue.isEmpty()) {
TreeNode node = queue.poll();
if (node == null) {
sb.append("null,");
} else {
sb.append(String.valueOf(node.val) + ",");
queue.offer(node.left);
queue.offer(node.right);
}
}
return sb.toString();
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
if (data.isEmpty()) return null;
String[] vals = data.split(",");
int[] nums = new int[vals.length];
TreeNode[] nodes = new TreeNode[vals.length];
for (int i = 0; i < vals.length; i++) {
if (i > 0) {
nums[i] = nums[i - 1];
}
if (vals[i].equals("null")) {
nodes[i] = null;
nums[i]++;
} else {
nodes[i] = new TreeNode(Integer.parseInt(vals[i]));
}
}
for (int i = 0; i < vals.length; i++) {
if (nodes[i] == null) {
continue;
}
nodes[i].left = nodes[2 * (i - nums[i]) + 1];
nodes[i].right = nodes[2 * (i - nums[i]) + 2];
}
return nodes[0];
}
}