golang刷leetcode 二叉树（1）不同二叉搜索树

输入: 3
输出:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
解释:
以上的输出对应以下 5 种不同结构的二叉搜索树：

   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 {
     var t  []*TreeNode
    if n<1{
        return t
    }
    return bst(1,n)
}

func bst(start,end int)[]*TreeNode{
    var t  []*TreeNode
    if end<start{
        return t
    }
    if start==end{
        t=append(t,&TreeNode{Val:start})
        return t
    }
    if start+1==end{
        t=append(t,&TreeNode{Val:start,Right:&TreeNode{Val:end}})
        t=append(t,&TreeNode{Val:end,Left:&TreeNode{Val:start}})
        return t
    }
    for i:=start;i<=end;i++{
        left:=bst(start,i-1)
        right:=bst(i+1,end)
        if len(left)<=0{
             for _,r:=range(right){
                root:=&TreeNode{Val:i,Right:r}
                t=append(t,root)
            }
        }else if len(right)<=0{
               for _,l:=range(left){
                root:=&TreeNode{Val:i,Left:l}
                t=append(t,root)
            }
        }else{
        for _,l:=range(left){
            for _,r:=range(right){
                root:=&TreeNode{Val:i,Left:l,Right:r}
                t=append(t,root)
            }
        }
        }
    } 
    return t
}