C++ 代码

/**
 * 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 {
    int s= 0;
public:
    unordered_map<int,int>dir;
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        for(int  i = 0;i<inorder.size();i++)
        {
            dir[inorder[i]] = i;
        }
         return   recur(0,preorder.size()-1,preorder,inorder);
    }

    TreeNode * recur(int l,int r,vector<int>& preorder, vector<int>& inorder)
    {
        if(l>r)return NULL;

          TreeNode * root =new TreeNode(preorder[s++]);//l == root
        int k ;
        // for(int i = l;i<=r;i++)
        // {
        //     if(root->val==inorder[i])//找到中序遍历的根节点 
        //     {
        //         k = i; break;
        //     }
        // }
        k = dir[root->val];
        root->left=recur(l,k-1,preorder,inorder); //划分左子树右子树
           root->right=recur(k+1,r,preorder,inorder);
        return root;

    }
};