LeetCode Convert Sorted List to Binary Search Tree

Posted 2020-06-14 gavinfish

LeetCode解题之Convert Sorted List to Binary Search Tree

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原题

给定一个升序的单向链表，将它转化为高度平衡的二叉搜索树。

注意点：

• 同一个序列转化成的二叉搜索树可能有多种

例子:

输入: nums = 1->2->3

输出:

  2
 / \\
1   3

解题思路

这题就是 Convert Sorted Array to Binary Search Tree 的升级版，可以先把链表转化为列表再解答。如果直接用链表解决的话，可以看出链表的特点是从头到尾依次遍历，因为是递增的，所以也就是从小到大依次遍历。而二叉所搜树的中序遍历的结果就是一个递增的序列，所以只要按照树的中序遍历的方式来构造即可。

AC源码

# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None


# 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 sortedListToBST(self, head):
        """
        :type head: ListNode
        :rtype: TreeNode
        """
        node, length = head, 0
        while node:
            node = node.next
            length += 1
        self.curr = head
        return self._sortedListToBST(0, length - 1)

    def _sortedListToBST(self, left, right):
        if left > right:
            return None
        mid = (left + right) // 2
        left = self._sortedListToBST(left, mid - 1)
        root = TreeNode(self.curr.val)
        root.left = left
        self.curr = self.curr.next
        root.right = self._sortedListToBST(mid + 1, right)
        return root


if __name__ == "__main__":
    None

欢迎查看我的Github (https://github.com/gavinfish/LeetCode-Python) 来获得相关源码。