/**
 * 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 = preorder.size();
        for(int i = 0;i < n;++i)    pos[inorder[i]] = i;    //pos中记录元素在中序中的位置
        return dfs(preorder,inorder,0,n-1,0,n-1);
    }

    TreeNode* dfs(vector<int>& pre,vector<int>& in,int pl,int pr,int il,int ir){
        if(pl > pr) return NULL;
        //建树，找根节点，找左右子树，把左右树连接到根节点
        TreeNode* root = new TreeNode(pre[pl]); //根节点是先序中的第一个点
        int k = pos[pre[pl]] - il;  //中序遍历中左子树的长度

        root->left = dfs(pre,in,pl + 1,pl + k,il,il + k -1);
        root->right = dfs(pre,in,pl + k + 1,pr,il + k + 1,ir);
        return root;
    }
};