Convert Sorted Array to Binary Search Tree in Python

Suppose we have one sorted array A. We have to generate one height-balanced binary search. In this problem, a height-balanced binary tree is actually a binary tree in which the depth of the two subtrees of every node never differs by more than 1. Suppose the array is like [-10, -3, 0, 5, 9]. So one possible output will be like: [0, -3, 9, -10, null, 5]

To solve this, we will follow these steps.

• If A is empty, then return Null
• find the mid element, and make it root
• Divide the array into two sub-arrays, left part of the mid element, and right part of the mid element
• recursively perform the same task for the left subarray and right subarray.

Let us see the following implementation to get a better understanding −

Example

 Live Demo

class TreeNode:
   def __init__(self, data, left = None, right = None):
      self.data = data
      self.left = left
      self.right = right
def insert(temp,data):
   que = []
   que.append(temp)
   while (len(que)):
      temp = que[0]
      que.pop(0)
      if (not temp.left):
         if data is not None:
            temp.left = TreeNode(data)
         else:
            temp.left = TreeNode(0)
         break
      else:
         que.append(temp.left)
      if (not temp.right):
         if data is not None:
            temp.right = TreeNode(data)
      else:
         temp.right = TreeNode(0)
      break
   else:
      que.append(temp.right)
def make_tree(elements):
   Tree = TreeNode(elements[0])
   for element in elements[1:]:
      insert(Tree, element)
   return Tree
def height(root):
   if root is None:
      return 0
   else :
      # Compute the height of left and right subtree
      l_height = height(root.left)
      r_height = height(root.right)
      #Find the greater one, and return it
      if l_height > r_height :
         return l_height+1
      else:
         return r_height+1
def print_given_level(root, level):
   if root is None:
      return
   if level == 1:
      print(root.data,end = ',')
   elif level > 1 :
      print_given_level(root.left , level-1)
      print_given_level(root.right , level-1)
def level_order(root):
   print('[', end = '')
   h = height(root)
   for i in range(1, h+1):
      print_given_level(root, i)
   print(']')
class Solution(object):
   def sortedArrayToBST(self, nums):
      """
      :type nums: List[int]
      :rtype: TreeNode
      """
      if not nums:
         return None
      mid = nums[len(nums)//2]
      root = TreeNode(mid)
      root.left = self.sortedArrayToBST(nums[:len(nums)//2])
      root.right = self.sortedArrayToBST(nums[len(nums)//2 +1 :])
      return root
nums = [-10,-3,0,5,9]
ob1 = Solution()
bst = ob1.sortedArrayToBST(nums)
level_order(bst)

Input

nums = [-10,-3,0,5,9]

Output

[0,-3,9,-10,5,]