0095. Unique Binary Search Trees I I

# 95. Unique Binary Search Trees II#

## 题目 #

Given an integer n, generate all structurally unique BST’s (binary search trees) that store values 1 … n.

Example:

``````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
``````

## 解题思路 #

• 输出 1~n 元素组成的 BST 所有解。这一题递归求解即可。外层循环遍历 1~n 所有结点，作为根结点，内层双层递归分别求出左子树和右子树。

## 代码 #

``````
package leetcode

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

func generateBSTree(start, end int) []*TreeNode {
tree := []*TreeNode{}
if start > end {
tree = append(tree, nil)
return tree
}
for i := start; i <= end; i++ {
left := generateBSTree(start, i-1)
right := generateBSTree(i+1, end)
for _, l := range left {
for _, r := range right {
root := &TreeNode{Val: i, Left: l, Right: r}
tree = append(tree, root)
}
}
}
return tree
}

``````

Sep 6, 2020