Verify Preorder Serialization of a Binary Tree

One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as #.

     _9_
    /   \
   3     2
  / \   / \
 4   1  #  6
/ \ / \   / \
# # # #   # #

For example, the above binary tree can be serialized to the string "9,3,4,#,#,1,#,#,2,#,6,#,#", where # represents a null node.

Given a string of comma separated values, verify whether it is a correct preorder traversal serialization of a binary tree. Find an algorithm without reconstructing the tree.

Each comma separated value in the string must be either an integer or a character '#' representing null pointer.

You may assume that the input format is always valid, for example it could never contain two consecutive commas such as "1,,3".

Example 1:

"9,3,4,#,#,1,#,#,2,#,6,#,#"

Return true

Example 2:

"1,#"

Return false

Example 3:

"9,#,#,1"

Return false

Notes:

1. Use stack: two "#" leaves remove one root
2. Use enter/exit degree?

    public boolean isValidSerialization(String preorder) {
        if (preorder == null) {
            return false;
        }

        preorder.trim();
        if (preorder.isEmpty()) {
            return true;
        }

        String[] nodes = preorder.split(",");
        return isValidPreorder(nodes);
    }

    private boolean isValidPreorder(String[] nodes) {
        Stack<String> s = new Stack<>();
        for (int i = 0; i < nodes.length; ++i) {
            if (nodes[i].equals("#")) {
                while(!s.isEmpty() && s.peek().equals("#")) {
                    s.pop();
                    if (s.isEmpty()) {
                        return false;
                    }
                    s.pop();
                }
            }
            s.push(nodes[i]);
        }

        return s.size() == 1 && s.peek().equals("#");
    }