/**
 * 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:
    void Build(int l1,int l2, int r1,int r2, const vector<int>& pre, const vector<int> & in, TreeNode*& root){
        root = new TreeNode(pre[l1]);
        int i;
        for(i=r1; i<=r2; i++)
            if(in[i]==root->val)
                break;
        if(i==r1)
            root->left=nullptr;
        else
            Build(l1+1, l1+i-r1,r1, i-1 ,pre,in, root->left);
        if(i==r2)
            root->right=nullptr;
        else
            Build(l1+i-r1+1, l2, i+1, r2, pre, in, root->right);
        return;
    }
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        if(preorder.size()==0 && inorder.size()==0)
            return nullptr;
        TreeNode* root;
        Build(0,preorder.size()-1, 0, inorder.size()-1, preorder, inorder, root);
        return root;
    }
};