95. 不同的二叉搜索树 II

原题

https://leetcode.cn/problems/unique-binary-search-trees-ii/description/

思路

递归

复杂度

时间：O(n³)
空间：O(n³)

Python代码

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def generateTrees(self, n: int) -> List[Optional[TreeNode]]:
        def backtrack(start, end):
            if start > end:
                return [None]
            res = []
            for i in range(start, end + 1):
                left_trees = backtrack(start, i-1)
                right_trees = backtrack(i+1, end)

                for l in left_trees:
                    for r in right_trees:
                        curr_tree = TreeNode(i)
                        curr_tree.left = l
                        curr_tree.right = r 
                        res.append(curr_tree)
            return res 
        return backtrack(1, n)
       

Go代码

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func generateTrees(n int) []*TreeNode {
	// 匿名函数
	var backtrack func(int, int) []*TreeNode
	backtrack = func(start int, end int) []*TreeNode {
		if start > end {
			return []*TreeNode{nil}
		}
		res := []*TreeNode{}
		for i := start; i <= end; i++ {
			left_trees := backtrack(start, i-1)
			right_trees := backtrack(i+1, end)
			for _, l := range left_trees {
				for _, r := range right_trees {
					curr_tree := &TreeNode{Val: i}
					curr_tree.Left = l
					curr_tree.Right = r
					res = append(res, curr_tree)
				}
			}
		}
		return res
	}
	return backtrack(1, n)
}