1. Description

　　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

2. Answer

public class Solution {

   public static boolean isValidSerialization(String preorder) {

       String[] strs = preorder.split(",");

        int degree = -1; // root has no indegree, for compensate init with -1

        for (String str : strs) {

            degree++; // all nodes have 1 indegree (root compensated)

            if (degree > 0) { // total degree should never exceeds 0

                return false;

            }

            if (!str.equals("#")) {// only non-leaf node has 2 outdegree

                degree -= 2;

            }

        }

        return degree == 0;

   }

}