Verify preorder traversal binary tree

// Given a string of comma-separated values preorder,
// return true if it is a correct preorder traversal serialization of a binary tree.

/* Explanation

In a binary tree, if we consider null as leaves, then

All non-null node provides 2 outdegree and 1 indegree (2 children and 1 parent), except root
All null node provides 0 outdegree and 1 indegree (0 child and 1 parent).

Suppose we try to build this tree. During building, we record diff = outdegree - indegree.
When the next node comes, we then decrease diff by 1, because the node provides an in degree.
If the node is not null, we increase diff by 2, because it provides two out degrees.

If a serialization is correct, diff should never be negative and diff will be zero when finished.
*/
public boolean isValidSerialization(String preorder) {
    String[] nodes = preorder.split(",");
    int diff = 1;
    for (String node: nodes) {
        if (--diff < 0) return false;
        if (!node.equals("#")) diff += 2;
    }
    return diff == 0;
}