<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<int> parsedPreorder(string& s) {
        vector<int>pre;
        for(int i=0;i<s.size();i++) {
            int cur = 0;
            while(i!=s.size() && s[i]!=',') {
                if(s[i]=='#') cur =-1;
                else cur= cur*10 + s[i]-'0';
                i++;
            }

            pre.push_back(cur);
        }
        return pre;
    }
    bool isValidSerialization(string preorder) {
        vector<int>stack;

        vector<int>pre = parsedPreorder(preorder);
        for(auto& curElement: pre) {
            stack.push_back(curElement);
        // stack size >=3 ok last two elements are #,then i can remove last three element 
        // and replace it with #
            while(stack.size()>=3 && stack.back()==-1 && stack[stack.size()-2]==-1 && stack[stack.size()-3]!=-1){
                stack.pop_back();
                stack.pop_back();
                // number is poped
                stack.pop_back();
                stack.push_back(-1);
            }
        }
        for(auto & it : stack)cout<<it<<" ";
        return stack.size()==1 && stack.back()==-1;
    }
};
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