<p>Given two integer arrays <code>inorder</code> and <code>postorder</code> where <code>inorder</code> is the inorder traversal of a binary tree and <code>postorder</code> is the postorder 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> inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
<strong>Output:</strong> [3,9,20,null,null,15,7]
</pre>

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

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

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

<ul>
	<li><code>1 &lt;= inorder.length &lt;= 3000</code></li>
	<li><code>postorder.length == inorder.length</code></li>
	<li><code>-3000 &lt;= inorder[i], postorder[i] &lt;= 3000</code></li>
	<li><code>inorder</code> and <code>postorder</code> consist of <strong>unique</strong> values.</li>
	<li>Each value of <code>postorder</code> also appears in <code>inorder</code>.</li>
	<li><code>inorder</code> is <strong>guaranteed</strong> to be the inorder traversal of the tree.</li>
	<li><code>postorder</code> is <strong>guaranteed</strong> to be the postorder 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:
    TreeNode* build(const vector<int>& inorder, const vector<int>& postorder, unordered_map<int,int>& inorder_rev, int inorder_left, int inorder_right) {
        int root = postorder[postorder.size()-1];
        int root_pos_inorder = inorder_rev[root];
        vector<int> postorder_left, postorder_right;
        for (int i = 0; i < postorder.size()-1; ++i) {
            int val = postorder[i];
            int pos_inorder = inorder_rev[val];
            if (pos_inorder < inorder_left || pos_inorder > inorder_right) {
                continue;
            }
            if (pos_inorder < root_pos_inorder) {
                postorder_left.push_back(val);
            }
            else {
                postorder_right.push_back(val);
            }
        }
        TreeNode* node = new TreeNode(root);
        if (!postorder_left.empty()) {
            node->left = build(inorder, postorder_left, inorder_rev, inorder_left, root_pos_inorder-1);
        }
        if (!postorder_right.empty()) {
            node->right = build(inorder, postorder_right, inorder_rev, root_pos_inorder+1, inorder_right);
        }
        return node;
    }
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        unordered_map<int,int> inorder_rev;
        for (int i = 0; i < inorder.size(); ++i) {
            inorder_rev[inorder[i]] = i;
        }
        return build(inorder, postorder, inorder_rev, 0, inorder.size() - 1);
    }
};
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