【LeetCode】Verify Preorder Serialization of a Binary Tree（331）

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;
   }
}