【朱迪的LeetCode刷题笔记】108. Convert Sorted Array to Binary Search Tree #Easy

Judie

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

Height-Balanced
A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

Example 1:
Input: nums = [-10,-3,0,5,9]
Output: [0,-3,9,-10,null,5]
Explanation: [0,-10,5,null,-3,null,9] is also accepted:

Example 2:
Input: nums = [1,3]
Output: [3,1]
Explanation: [1,null,3] and [3,1] are both height-balanced BSTs.


Constraints:
1 <= nums.length <= 104
-104 <= nums[i] <= 104
nums is sorted in a strictly increasing order.

Judy
Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def sortedArrayToBST(self, nums):
        """
        :type nums: List[int]
        :rtype: TreeNode
        """

        def f(nums):
            l = len(nums)
            if l == 1:
                return TreeNode(val=nums[0])
            elif l == 2:
                return TreeNode(val=nums[1], left=TreeNode(val=nums[0]))
            elif l == 3:
                return TreeNode(val=nums[1], left=TreeNode(val=nums[0]), right=TreeNode(val=nums[2]))
            m = l // 2
            return TreeNode(val=nums[m], left=f(nums[:m]), right=f(nums[m+1:]))
        
        return f(nums)