95.md

95. Unique Binary Search Trees II

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

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

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

my thoughts:

1. recursion

my solution:

**********72ms
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def generateTrees(self, n):
        """
        :type n: int
        :rtype: List[TreeNode]
        """
        
        def genTree(start, end):
            res = []
            if start == end:
                return None
            for i in range(start, end):
                #print(i, start, end)
                left = genTree(start, i)
                right = genTree(i+1, end)
                for lt in left or [None]:
                    for rt in right or [None]:
                        root = TreeNode(i)
                        root.left, root.right = lt, rt
                        res.append(root)
            return res
        
        return genTree( 1, n+1 ) or []

my comments:

from other ppl's solution:

1. N/A