331. Verify Preorder Serialization of a Binary Tree

Medium
One way to serialize a binary tree is to use preorder 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 '#'.
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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 preorder, return true if it is a correct preorder traversal serialization of a binary tree.
It is guaranteed that 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".
Note: You are not allowed to reconstruct the tree.
Example 1:
Input: preorder = "9,3,4,#,#,1,#,#,2,#,6,#,#"
Output: true
Example 2:
Input: preorder = "1,#"
Output: false
Example 3:
Input: preorder = "9,#,#,1"
Output: false
Constraints:
  • 1 <= preorder.length <= 10^4
  • preorder consist of integers in the range [0, 100] and '#' separated by commas ','.

解題

使用 indegree, outdegree 來做判斷,參考討論區這個超神的解答​
Runtime: 0 ms, faster than 100%
Memory Usage: 2.2 MB, less than 93.55%
func isValidSerialization(preorder string) bool {
degree := -1
for i:=0; i<len(preorder); i++ {
if preorder[i] == ',' { continue }
if i > 0 && preorder[i - 1] >= '0' && preorder[i - 1] <= '9' { continue }
degree++
if degree > 0 { return false }
if preorder[i] != '#' {
degree -= 2
}
}
return degree == 0
}