Leetcode 95.不同的二叉搜索树II

Posted 2021-11-27 jawen

给定一个整数 n，生成所有由 1 ... n 为节点所组成的二叉搜索树。

示例:

输入: 3
输出:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
解释:
以上的输出对应以下 5 种不同结构的二叉搜索树：

   1         3     3      2      1
    \       /     /      / \           3     2     1      1   3      2
    /     /       \                    2     1         2                 3

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<TreeNode*> generateTrees(int n) {
13         if(n == 0)
14         {
15             return vector<TreeNode*>{};
16         }
17         return helper(1,n);
18     }
19     vector<TreeNode*> helper(int begin,int end)
20     {
21         vector<TreeNode*> res;
22         if(begin > end)
23         {
24             res.push_back(NULL);
25             return res;
26         }
27         for(int i = begin; i<= end;++i)
28         {
29             vector<TreeNode*> left = helper(begin,i-1);
30             vector<TreeNode*> right = helper(i+1,end);
31             for(auto l : left)
32                 for(auto r: right)
33                 {
34                     TreeNode* root = new TreeNode(i);
35                     root-> left = l;
36                     root-> right =  r;
37                     res.push_back(root);
38                 }
39         }
40         return res;
41     }
42 };