0331-Verify Preorder Serialization of a Binary Tree.md

Longest Increasing Path in a Matrix

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

Solution

O(preorder) Time, O(1) Space:

class Solution {
    func isValidSerialization(_ preorder: String) -> Bool {
        let preorder = preorder.split(separator: ",")
        var edges = 1
        for string in preorder {
            edges-=1
            guard edges >= 0 else { return false }
            switch string {
                case "#":
                break
                default:
                edges+=2
            }
        }
        return edges == 0
    }
}