Leetcode 331: 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

 

public class Solution {
    public bool IsValidSerialization(string preorder) {
        if (preorder.Length < 1) return true;
        
        var nodes = preorder.Split(',').ToList();        
        
        return  Deserialize(nodes) && nodes.Count == 0;
    }
    
    private bool Deserialize(IList<string> nodes)
    {
        if (nodes.Count == 0) return true;
        
        var n = nodes[0];
        nodes.RemoveAt(0);
        
        if (n == "#") return true;
        if (nodes.Count < 2) return false;    
        
        return (Deserialize(nodes) && Deserialize(nodes));
    }
}