<p>Given two integer arrays <code>preorder</code> and <code>inorder</code> where <code>preorder</code> is the preorder traversal of a binary tree and <code>inorder</code> is the inorder traversal of the same tree, construct and return <em>the binary tree</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/tree.jpg" style="width: 277px; height: 302px;" />
<pre>
<strong>Input:</strong> preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
<strong>Output:</strong> [3,9,20,null,null,15,7]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> preorder = [-1], inorder = [-1]
<strong>Output:</strong> [-1]
</pre>

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

<ul>
	<li><code>1 &lt;= preorder.length &lt;= 3000</code></li>
	<li><code>inorder.length == preorder.length</code></li>
	<li><code>-3000 &lt;= preorder[i], inorder[i] &lt;= 3000</code></li>
	<li><code>preorder</code> and <code>inorder</code> consist of <strong>unique</strong> values.</li>
	<li>Each value of <code>inorder</code> also appears in <code>preorder</code>.</li>
	<li><code>preorder</code> is <strong>guaranteed</strong> to be the preorder traversal of the tree.</li>
	<li><code>inorder</code> is <strong>guaranteed</strong> to be the inorder traversal of the tree.</li>
</ul>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    pair<vector<int>, vector<int>> findNeighbours(const vector<int>& v, const int&purpose)
    {
        vector<int> left;
        vector<int> right;

        bool wasPurpose = false;

        for (auto&i:v)
        {
            if (i == purpose)
            {
                wasPurpose = true;
                continue;
            }

            if (!wasPurpose)
            {
                left.emplace_back(i);
            }
            else
            {
                right.emplace_back(i);
            }
        }
        return make_pair(left, right);
    }

    vector<int> preorder;
    int counter = 0;

    TreeNode* builder(vector<int>& inorder) {
        if (counter >= preorder.size()) { return nullptr; };
        if (inorder.empty()) { return nullptr; }

        int val = preorder[counter];
        counter++;

        auto neighbours = findNeighbours(inorder, val);     

        TreeNode* node = new TreeNode(val);
        node->left = builder(neighbours.first);
        node->right = builder(neighbours.second);

        return node;
    }

    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        this->preorder = preorder;
        return builder(inorder);
    }
};
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