class Solution {
public:
    int rootid = 0;
    unordered_map<int,int> hashmap;
    TreeNode* preorder(int i_left,int i_right,
    vector<int>& inorder, vector<int>& postorder)
    {
        if(i_left > i_right)
        return nullptr;

        int root_val = postorder[rootid];
        TreeNode* root = new TreeNode(root_val);

        int tmp = hashmap[root_val];

        rootid--;

        root->right = preorder(tmp+1,i_right,inorder,postorder);
        root->left = preorder(i_left,tmp-1,inorder,postorder);

        return root;
    }
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        rootid = (int)postorder.size()-1;

        int id = 0;
        for(auto& num : inorder)
        {
            hashmap[num] = id++;
        }

        return preorder(0,(int)inorder.size()-1,inorder,postorder);
    }
};