<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* construct(int inStart, int inEnd, int postStart, int postEnd,
        vector<int>& inorder, vector<int>& postorder, vector<int>& inOrderIdx) {
        if (inStart > inEnd || postStart > postEnd) {
            return nullptr;
        }
        // root is at postEnd
        TreeNode* root = new TreeNode(postorder[postEnd]);
        int idx = inOrderIdx[root->val + 3000];
        int rightLength = inEnd - idx;
        // cout << postStart << ' ' << postEnd << ' ' << rightLength << '\n';
        // assert(rightLength >= 0);
        // elements to left of root in order are left subtree
        root->left = construct(inStart, idx - 1, postStart,
            postEnd - rightLength - 1, inorder, postorder, inOrderIdx);
        // elements to right in order are right subtree
        root->right = construct(idx + 1, inEnd, postEnd - rightLength,
            postEnd - 1, inorder, postorder, inOrderIdx);
        return root;
    }

    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        int n = inorder.size();
        vector<int> inOrderIdx(6001, -1);
        for (int i = 0; i < n; i++) {
            inOrderIdx[inorder[i] + 3000] = i;
        }
        return construct(0, n-1, 0, n-1, inorder, postorder, inOrderIdx);
    }
};
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