LeetCode : 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

先求出子问题的解。

/**
* 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 *> generateTrees(int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<TreeNode *> res;
generate(res, 1, n);
return res;

}
void generate(vector<TreeNode *> &res, int start, int end){
if(start > end){
res.push_back(NULL);
return;
}
for(int i = start; i<= end; ++i){
vector<TreeNode *> left;
vector<TreeNode *> right;
generate(left, start, i - 1);
generate(right, 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];
res.push_back(root);
}
}
}
}
};