LeetCode 重建BST

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 struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  };
class Solution {
public:
   vector<TreeNode*> solve(int start, int end)
{
vector<TreeNode*> vectree;
if (start > end)
{
vectree.push_back(nullptr);
return vectree;
}
if (start == end)
{
vectree.push_back(new TreeNode(start));
return vectree;
}
TreeNode* root = nullptr;
for (int k = start; k <= end; k++)
{
vector<TreeNode*> vecleft = solve(start, k-1);
vector<TreeNode*> vecright = solve(k + 1, end);
for (TreeNode* left : vecleft)
{
for (TreeNode* right : vecright)
{
root = new TreeNode(k);
root->left = left;
root->right = right;
                    vectree.push_back(root);
}
}

}
return vectree;
}
vector<TreeNode *> generateTrees(int n) {
vector<TreeNode*> vectree;
if (n < 0)
return vectree;
vectree = solve(1, n);
return vectree;
}
};