leetcode[95]Unique Binary Search Trees II

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
void creatTree(vector<TreeNode *> &res,int start, int end)
{
  if(start>end)
  {
      res.push_back(NULL);
      return;
  }
  for(int k=start;k<=end;k++)
  {
      vector<TreeNode *> left;
      vector<TreeNode *> right;
      creatTree(left,start,k-1);
      creatTree(right,k+1,end);
      for(int i=0;i<left.size();i++)
      {
          for(int j=0;j<right.size();j++)
          {
              TreeNode *root=new TreeNode(k);
              root->left=left[i];
              root->right=right[j];
              res.push_back(root);
          }
      }
  }
  return;
}
    vector<TreeNode *> generateTrees(int n) {
    vector<TreeNode *> res;
    creatTree(res,1,n);
    return res;
    }
};