从前序与中序遍历序列构造二叉树

题目：

/**
 * 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:
    int Search(int val,int &l,int &r) {
        for(int i = l;i = n || l > r) return;
        root = new TreeNode(preorder[pos]);
        int id = Search(preorder[pos++],l,r);
        dfs(root->left,l,id-1);
        dfs(root->right,id+1,r);
    }
    TreeNode* buildTree(vector& preorder, vector& inorder) {
        this->preorder = preorder;
        this->inorder = inorder;
        n = inorder.size();
        if(!n) return nullptr;
        pos = 0;
        TreeNode *root;
        dfs(root,0,n-1);
        return root;
    }
private:
    vector preorder;
    vector inorder;
    int pos,n;
};