Unique Binary Search Trees II
Given 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
/**
* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
//找到一个数作为根结点,剩余的数分别划入左子树或者右子树
vector<TreeNode *> generateTrees(int n) {
return create(1,n);
}
vector<TreeNode *>create(int start,int end)
{
vector<TreeNode *> res;
if(start>end)
{
res.push_back(NULL);
return res;
}
for(int i=start;i<=end;i++)
{
vector<TreeNode *>left=create(start,i-1);
vector<TreeNode *>right=create(i+1,end);
for(int j=0;j<left.size();j++)
{
for(int k=0;k<right.size();k++)
{
TreeNode *r=new TreeNode(i);
r->left=left[j];
r->right=right[k];
res.push_back(r);
}
}
}
return res;
}
};