LeetCode OJ:Unique Binary Search Trees II

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

confused what

"{1,#,2,3}"

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

算法思想：

DFS罗列所有可能情况：

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