331. Verify Preorder Serialization of a Binary Tree

Problem

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 <= 10^4

• preorder consist of integers in the range [0, 100] and '#' separated by commas ','.

Solution

class Solution {
    public boolean isValidSerialization(String preorder) {
        int count = 1;
        int length = preorder.length();
        for (int i = 1; i <= length; i++) {
            if (i == length || preorder.charAt(i) == ',') {
                --count;
                if (count < 0) {
                    return false;
                }
                count += preorder.charAt(i - 1) == '#' ? 0 : 2;
            }
        }
        return count == 0;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).