<p>You have <code>n</code> binary tree nodes numbered from <code>0</code> to <code>n - 1</code> where node <code>i</code> has two children <code>leftChild[i]</code> and <code>rightChild[i]</code>, return <code>true</code> if and only if <strong>all</strong> the given nodes form <strong>exactly one</strong> valid binary tree.</p>

<p>If node <code>i</code> has no left child then <code>leftChild[i]</code> will equal <code>-1</code>, similarly for the right child.</p>

<p>Note that the nodes have no values and that we only use the node numbers in this problem.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/08/23/1503_ex1.png" style="width: 195px; height: 287px;" />
<pre>
<strong>Input:</strong> n = 4, leftChild = [1,-1,3,-1], rightChild = [2,-1,-1,-1]
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/08/23/1503_ex2.png" style="width: 183px; height: 272px;" />
<pre>
<strong>Input:</strong> n = 4, leftChild = [1,-1,3,-1], rightChild = [2,3,-1,-1]
<strong>Output:</strong> false
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/08/23/1503_ex3.png" style="width: 82px; height: 174px;" />
<pre>
<strong>Input:</strong> n = 2, leftChild = [1,0], rightChild = [-1,-1]
<strong>Output:</strong> false
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == leftChild.length == rightChild.length</code></li>
	<li><code>1 &lt;= n &lt;= 10<sup>4</sup></code></li>
	<li><code>-1 &lt;= leftChild[i], rightChild[i] &lt;= n - 1</code></li>
</ul>

class Solution {
public:
    bool validateBinaryTreeNodes(int n, vector<int>& leftChild, vector<int>& rightChild) {
        unordered_set<int> st; //child
        // st.insert(0);
        // for(int i=0;i<n;i++){
        //     if(leftChild[i] != -1){
        //         if(st.find(leftChild[i]) != st.end()){
        //             return false;
        //         }else{
        //             st.insert(leftChild[i]);
        //         }
        //     }

        //     if(rightChild[i] != -1){
        //         if(st.find(rightChild[i]) != st.end()){
        //             return false;
        //         }else{
        //             st.insert(rightChild[i]);
        //         }
        //     }  
        // }

        for(int i=0;i<n;i++){
            if(leftChild[i] != -1) st.insert(leftChild[i]);
            if(rightChild[i] != -1) st.insert(rightChild[i]);
        }
        int root = -1;
        bool flag2 = false;
        for(int i=0;i<n;i++){
            if(st.find(i) == st.end()){
                if(flag2) return false;
                root = i;
                flag2 = true;
            }
        }

        if(root == -1) return false;
        // cout<<root<<endl;
        queue<int> q;
        q.push(root);
        vector<bool> visited(n,false);
        visited[root] = true;
        st.clear();
        while(!q.empty()){
            int m = q.size();
            for(int i=0;i<m;i++){
                int curr = q.front();
                q.pop();
                // cout<<curr<<" ";
                // if(curr != -1) visited[curr] = true;
                if(leftChild[curr] != -1){
                    if(visited[leftChild[curr]]) return false;
                    visited[leftChild[curr]] = true;
                    q.push(leftChild[curr]);
                }
                if(rightChild[curr] != -1) {
                    if(visited[rightChild[curr]]) return false;
                    visited[rightChild[curr]] = true;
                    q.push(rightChild[curr]);
                }
            }
            cout<<endl;
        }
        visited[0] = true;

        for(int i=0;i<n;i++){
            if(!visited[i]) return false;
        }

        return true;

    }
};
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