[95]Unique Binary Search Trees II

【题目描述】

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

confused what 

"{1,#,2,3}"

 means? >
read more on how binary tree is serialized on OJ.

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【思路】

用递归实现的，思路基本和Unique Binary Search Trees一样。

【代码】

/**
* 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*> generateTrees(int n) {
if(n==0) return generate(1,0);
else return generate(1,n);
}
vector<TreeNode*> generate(int start,int end){
vector<TreeNode*> subtree;
if(start>end){
subtree.push_back(NULL);
return subtree;
}
for(int i=start;i<=end;i++){
vector<TreeNode*> lefttree=generate(start,i-1);
vector<TreeNode*> righttree=generate(i+1,end);
for(int j=0;j<lefttree.size();j++){
for(int k=0;k<righttree.size();k++){
TreeNode* node=new  TreeNode(i);
node->left=lefttree[j];
node->right=righttree[k];
subtree.push_back(node);
}
}
}
return subtree;
}
};