Unique Binary Search Trees II

题目：

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.

题目：

这题要求将每个子树输出，思想就是对于n，以1-n为根节点，然后左右子树分别是比其小和比其大的节点构成的，对左右子树的的形态也要使用遍历，代码中使用的是foreach遍历：for(i:j)这种形式

/**
* 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);
return generate(1, n);
}
private:
vector<TreeNode *> generate(int start, int end) {
vector<TreeNode*> subTree;
if (start > end) {
subTree.push_back(nullptr);
return subTree;
}
for (int k = start; k <= end; k++) {
vector<TreeNode*> leftSubs = generate(start, k - 1);
vector<TreeNode*> rightSubs = generate(k + 1, end);
for (auto i : leftSubs) {
for (auto j : rightSubs) {
TreeNode *node = new TreeNode(k);
node->left = i;
node->right = j;
subTree.push_back(node);
}
}
}
return subTree;
}
};