[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

这道题思路和第一题其实有些类似。第一题是利用动态规划的思想，选取一个数作为根节点，然后分别计算左子树和右子树的数量，递归计算，得出最后结果。

这道题可以选取某个节点作为根节点，然后分别生成左子树和有字数的集合，然后把根节点和左子树、右子树的集合组合起来就得到所有的BST了

class Solution {
public:
vector<TreeNode *> generateTrees(int n) {
return generate(1, n);

}
vector<TreeNode *> generate(int begin,int end){
vector<TreeNode *>result;
if (begin > end){
result.push_back(NULL);
return result;
}
for (int i = begin; i <= end; i++){
vector<TreeNode *> left = generate(begin, i - 1);
vector<TreeNode *> right = generate( i + 1,end);
for (int m = 0; m < left.size(); m++){
for (int n = 0; n < right.size(); n++){
TreeNode *newNode = new TreeNode(i);
newNode->left = left[m];
newNode->right = right
;
result.push_back(newNode);
}
}
}
return result;
}
};