Leetcode 95 Unique Binary Search Trees II

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,

Given n = 3, your program should return all 5 unique BST's shown below.

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

求出所有二叉排序数的情况。
每一层枚举中间的点，将小于它的放在左子树，大于的放在右子树，没有元素的子树记得指向NULL。

/**
* 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:
vector<TreeNode*> dfs(int l,int r)
{
if(l==r)
{
TreeNode* p=new TreeNode(l);
vector<TreeNode*> now(1,p);
return now;
}
vector<TreeNode*> res;
for(int mid=l;mid<=r;mid++)
{
vector<TreeNode*> left,right;
if(l<=mid-1)
left=dfs(l,mid-1);
else
left.push_back(NULL);//开始忘记了
if(mid+1<=r)
right=dfs(mid+1,r);
else
right.push_back(NULL);
for(int i=0;i<left.size();i++)
{
for(int j=0;j<right.size();j++)
{
TreeNode* p=new TreeNode(mid);
p->left=left[i];
p->right=right[j];
res.push_back(p);
}
}
}
return res;
}
vector<TreeNode*> generateTrees(int n) {
vector<TreeNode*> res,next;
res=dfs(1,n);
return res;
}
};