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You can send the tree as preorder and in order traversals or XML tree and then parse to reconstruct the tree.

Send binary tree as an array.

A binary tree can be put in the array very easily. child of nodes of "i" are located at "2*i" and "2*i+1". So just pass him as a stream of numbers starting from the array index 1. You have to send the first number to be the number of nodes in the tree, so that server can allocate the array.

As mentioned any two traversal can be sent for syncing details between client and server.

I would send to the server in the in-order format {{1, 2, {3, 4, null}}, 5, {7, {null, 8, 6}, 9}}, there will be 3 elements in the curly brace representing left, root and right, you can as well put them as left, root, right for easier construction of tree on the server. On the server we would construct a tree using the java class, so that it will be easier for other operations on the tree.

List<Character> serialization(Node root) {
List<Character> list=new ArrayList<Character>();
if(root==null) {
list.add('#');
} else {
list.add(root.val);
list.addAll(serialization(root.left));
list.addAll(serialization(root.right));
}
return list;
}

Node deSerialization(List<Character> list) {
char c=list.get(0);
list.remove(0);
if(c=='#') return null;
Node root=new Node(c);
root.left=deSerialization(list);
root.right=deSerialization(list);
return root;
}