<p>One way to serialize a binary tree is to use <strong>preorder traversal</strong>. When we encounter a non-null node, we record the node&#39;s value. If it is a null node, we record using a sentinel value such as <code>&#39;#&#39;</code>.</p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/03/12/pre-tree.jpg" style="width: 362px; height: 293px;" />
<p>For example, the above binary tree can be serialized to the string <code>&quot;9,3,4,#,#,1,#,#,2,#,6,#,#&quot;</code>, where <code>&#39;#&#39;</code> represents a null node.</p>

<p>Given a string of comma-separated values <code>preorder</code>, return <code>true</code> if it is a correct preorder traversal serialization of a binary tree.</p>

<p>It is <strong>guaranteed</strong> that each comma-separated value in the string must be either an integer or a character <code>&#39;#&#39;</code> representing null pointer.</p>

<p>You may assume that the input format is always valid.</p>

<ul>
	<li>For example, it could never contain two consecutive commas, such as <code>&quot;1,,3&quot;</code>.</li>
</ul>

<p><strong>Note:&nbsp;</strong>You are not allowed to reconstruct the tree.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> preorder = "9,3,4,#,#,1,#,#,2,#,6,#,#"
<strong>Output:</strong> true
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> preorder = "1,#"
<strong>Output:</strong> false
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> preorder = "9,#,#,1"
<strong>Output:</strong> false
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= preorder.length &lt;= 10<sup>4</sup></code></li>
	<li><code>preorder</code> consist of integers in the range <code>[0, 100]</code> and <code>&#39;#&#39;</code> separated by commas <code>&#39;,&#39;</code>.</li>
</ul>

class Solution {
public:
    vector<string> split(string x) {
        vector<string> ret;
        string temp;
        for(int i = 0; i < (int) x.size(); i++) {
            temp = "";
            while(i < (int) x.size() && x[i] != ',') {
                temp += x[i];
                i++;
            }
            ret.push_back(temp);
        }
        return ret;
    }

    bool isValidSerialization(string preorder) {
        if(preorder == "#") return true;
        vector<string> vec = split(preorder);
        stack<int> nodes;
        if(vec[0] != "#") {
            nodes.push(2);
        }

        for(int i = 1; i < (int) vec.size(); i++) {
            while(!nodes.empty() && nodes.top() == 0) nodes.pop();
            if(nodes.empty()) return false;
            // stack<int> copy = nodes;
            // while(!copy.empty()) {
            //     cout << copy.top() << " ";
            //     copy.pop();
            // }
            // cout << "\n";
            int topNodes = nodes.top(); nodes.pop();
            if(vec[i] == "#") {
                nodes.push(topNodes - 1);
            } else {
                nodes.push(topNodes - 1);
                nodes.push(2);
            }
        }
        while(!nodes.empty() && nodes.top() == 0) nodes.pop();
        if(!nodes.empty()) return false;
        
        return true;
    }
};
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