[leetcode刷题系列]Unique Binary Search Trees II

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
vector<TreeNode *> dfs(int left, int right){
vector<TreeNode *> ret;
if(left > right){
ret.push_back(0);
return ret;
}
for(int i = left; i <= right; ++i){
vector<TreeNode *> left_ret = dfs(left, i - 1);
vector<TreeNode *> right_ret = dfs(i + 1, right);
for(int ii = 0; ii < left_ret.size(); ++ ii)
for(int jj = 0; jj < right_ret.size(); ++ jj){
TreeNode * root = new TreeNode(i);
root->left = left_ret[ii];
root->right = right_ret[jj];
ret.push_back(root);
}
}
return ret;
}
public:
vector<TreeNode *> generateTrees(int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
return dfs(1, n);
}
};