95 Unique Binary Search Trees II

🔀 Pick One

Given an integer n_, return all the structurally unique BST's (binary search trees), which has exactly n nodes of unique values from 1 to n_. Return the answer in any order.

Example 1:

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

Input: n = 3
Output: [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]]

Example 2:

Input: n = 1
Output: [[1]]

Constraints:

  • 1 <= n <= 8
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# 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 build_tree(self, lo: int, hi: int) -> List[Optional[TreeNode]]:
        if lo > hi:
            return [None]
        trees = []
        for v in range(lo, hi+1):
            for l in self.build_tree(lo, v-1):
                for r in self.build_tree(v+1, hi):
                    trees.append(TreeNode(v, l, r))
        return trees

    def generateTrees(self, n: int) -> List[Optional[TreeNode]]:
        return self.build_tree(1, n)