/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    unordered_map<int,int> pos;

    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int n = inorder.size();
        for(int i = 0;i < n;i++){
            pos[inorder[i]] = i;
        }
        return dfs(preorder,inorder,0,n-1,0,n-1);
    }
    TreeNode* dfs(vector<int>& preorder,vector<int>& inorder,int pl,int pr,int il,int ir){
        if(pl>pr)return NULL;
        int k = pos[preorder[pl]] - il;
        TreeNode* root = new TreeNode(preorder[pl]);
        root->left = dfs(preorder,inorder,pl+1,pl+k,il,il+k-1);
        root->right = dfs(preorder,inorder,pl+k+1,pr,il+k+1,ir);
        return root;
    }
};