leetcode 95 所有的二叉排序樹

首先是遞歸的解法，

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def tree(self,left,right):
        
        if right<left:
            return [None]
        root_list=[]
        for i in range(left,right+1):
            left_set=self.tree(left,i-1)
            right_set=self.tree(i+1,right)
            for l in left_set:
                for r in right_set:
                    root=TreeNode(i)
                    root.left,root.right=l,r
                    root_list.append(root)
        return root_list
    def generateTrees(self, n: int) -> List[TreeNode]:
        if n==0:
            return []
        return self.tree(1,n)
        

dp解法

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
from collections import defaultdict
class Solution:
 
    def generateTrees(self, n: int) -> List[TreeNode]:        
        dp=defaultdict(list)
        for i in range(1,n+1):
            dp[(i,i)].append(TreeNode(i))
        
        for k in range(2,n+1):
         
            for i in range(1,n-k+2):

                for j in range(i,i+k):

                    if j-1<i:
                        dp[(i,j-1)]=[None]
                    if i+k-1<j+1:
                        dp[(j+1,i+k-1)]=[None]

                    for l in dp[(i,j-1)]:
                        for r in dp[(j+1,i+k-1)]:
                            root=TreeNode(j)
        

                            root.left,root.right=l,r
                            dp[(i,i+k-1)].append(root)

        return dp[(1,n)]