LeetCode in Kotlin
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 '#'.
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 <= 104preorderconsist of integers in the range[0, 100]and'#'separated by commas','.
Solution
class Solution {
fun isValidSerialization(preorder: String): Boolean {
var count = 1
val length = preorder.length
for (i in 1..length) {
if (i == length || preorder[i] == ',') {
--count
if (count < 0) {
return false
}
count += if (preorder[i - 1] == '#') 0 else 2
}
}
return count == 0
}
}