[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

哎，WA了两次，因为把root = new TreeNode(i)放错了位置，这句应该放在最里面的循环里。因为每次都要新生成一棵树！

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

vector<TreeNode *> generateTrees(int n) {
return generateTrees(1, n);
}
};