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;
    }
};