Unique Binary Search Trees II - LeetCode

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

思路：通过递归实现。我们设置一个生成节点编号st到ed的子树的递归函数，对于给定的st到ed，我们枚举中间的每一个数当作root节点，假设数为i，则继续调用参数为st到i-1以及i+1到ed的该函数生成所有左孩子和右孩子子树。

 /**
  * 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) {
         return gen_sub(, n);
     }
     vector<TreeNode*> gen_sub(int st, int ed)
     {
         vector<TreeNode*> res;
         if (st > ed)
             res.push_back(NULL);
         for (int i = st; i <= ed; i++)
         {
             vector<TreeNode*> lnodes = gen_sub(st, i - );
             vector<TreeNode*> rnodes = gen_sub(i + , ed);
             for (TreeNode *ln : lnodes)
                 for (TreeNode *rn : rnodes)
                 {
                     TreeNode *root = new TreeNode(i);
                     root->left = ln;
                     root->right = rn;
                     res.push_back(root);
                 }
         }
         return res;
     }
 };