[LeetCode]题解（python）：095-Unique Binary Search Trees II

题目来源：

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

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题意分析：

　　给一个整数，返回所有中序遍历是1到n的树。

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题目思路：

　　这可以用递归的思想。首先确定根节点，如果k是根节点，那么1-k-1是左子树，而k+1-n为右子树。

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代码（python）：

　　

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

class Solution(object):
    def generateTrees(self, n):
        """
        :type n: int
        :rtype: List[TreeNode]
        """
        if n == 0:
            return []
        def solve(begin,end):
            if begin > end:
                return [None]
            i = begin
            ans = []
            while i <= end:
                tmp1,tmp2 = solve(begin, i - 1),solve(i + 1,end)
                for j in tmp1:
                    for k in tmp2:
                        tmp = TreeNode(i)
                        tmp.left = j
                        tmp.right = k
                        ans.append(tmp)
                i += 1
            return ans
        return solve(1,n)