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: Input: "9,3,4,#,#,1,#,#,2,#,6,#,#" Output: true Example 2: Input: "1,#" Output: false Example 3: Input: "9,#,#,1" Output: false
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 the difference between out degree and in degree 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.
If you have different approach in mind or have any suggestion for this implementation feel free to share in the comment below. Thanks!