Tree - 95. Unique Binary Search Trees II

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

/**

 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func generateTrees(n int) []*TreeNode {
    if n == 0 {return nil}
    return genTree(1, n)
}


func genTree(start, end int) []*TreeNode {
    var list []*TreeNode
    
    if (start > end) {
        list = append(list, nil)
        return list
    }

    for i := start; i <= end; i++ {
        left := genTree(start, i - 1);
        right := genTree(i + 1,end);

        for _, lnode := range left {
            for _, rnode := range right {
                list = append(list, &TreeNode{
                    Val: i,
                    Left : lnode,
                    Right: rnode,
                })
            }
        }
    }
    return list
}