題目：
給定兩個整數陣列，分別表示二元樹的前序遍歷和中序遍歷結果，請構造該二元樹並回傳其根節點。

範例：

前序遍歷 (preorder) = [3, 9, 20, 15, 7]
中序遍歷 (inorder) = [9, 3, 15, 20, 7]

回傳的二元樹為：

    3
   / \
  9  20
    /  \
   15   7

實作：

class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        unordered_map<int, int> inorder_map;
        for (int i = 0; i < inorder.size(); i++) {
            inorder_map[inorder[i]] = i;
        }
        return buildTree(preorder, 0, preorder.size() - 1, inorder, 0, inorder.size() - 1, inorder_map);
    }

private:
    TreeNode* buildTree(vector<int>& preorder, int preStart, int preEnd, 
                        vector<int>& inorder, int inStart, int inEnd, 
                        unordered_map<int, int>& inorder_map) {
        if (preStart > preEnd || inStart > inEnd) return nullptr;

        int rootVal = preorder[preStart];
        TreeNode* root = new TreeNode(rootVal);

        int inRootIndex = inorder_map[rootVal];
        int numsLeft = inRootIndex - inStart;

        root->left = buildTree(preorder, preStart + 1, preStart + numsLeft, inorder, inStart, inRootIndex - 1, inorder_map);
        root->right = buildTree(preorder, preStart + numsLeft + 1, preEnd, inorder, inRootIndex + 1, inEnd, inorder_map);

        return root;
    }
};