<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:
    bool isValidSerialization(string preorder) {
        if(preorder=="#") {
            return true;
        }
        stringstream ss (preorder);
        string node;
        stack<pair<string,int>> st; //0->at root, 1->left, 2->right
        getline (ss, node, ',');
        if(node == "#") {
            return false;
        }
        st.push({node, 0}); 
        while(getline (ss, node, ',')) {
            if(st.empty()) {
                // no parent
                return false;
            }
            if(node != "#") {
                st.push({node, 0});
            } else {
                // at leaf now, pop to parent
                
                while(!st.empty()) {
                    string parent = st.top().first;
                    int hasLeftRight = st.top().second;
                    if(hasLeftRight==0) {
                        // finished left branch, go to right
                        st.top().second = 1;
                        break;
                    } else if (hasLeftRight==1) {
                        // finishedright branch
                        st.pop();
                    } 
                    
                }
            }
        }
        return st.empty();
    }
};
11500