Leetcode 95.不同的二叉搜索树II

不同的二叉搜索树2

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：

 /**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 class Solution {
 public:
     vector<TreeNode *> generateTrees(int n) {
         vector<TreeNode *> ret;
         if (n == 0)
             return ret;
         return Helper(1, n);
     }
     vector<TreeNode *> Helper(int begin, int end)
     {
         vector<TreeNode *> ret;
         if (begin > end)
             ret.push_back(NULL);
         else if (begin == end)
         {
             TreeNode* node = new TreeNode(begin);
             ret.push_back(node);
         }
         else
         {
             for (int i = begin; i <= end; i++)
             {//root
                 vector<TreeNode *> left = Helper(begin, i - 1);
                 vector<TreeNode *> right = Helper(i + 1, end);
                 for (int l = 0; l < left.size(); l++)
                 {
                     for (int r = 0; r < right.size(); r++)
                     {
                         //new tree
                         TreeNode* root = new TreeNode(i);
                         root->left = left[l];
                         root->right = right[r];
                         ret.push_back(root);
                     }
                 }
             }
         }
         return ret;
     }
 };