Find Common Ancestor of Two Elements in a Binary Tree Using Python

Suppose we have a binary tree, and we also have two numbers a and b, we have to find the value of the lowest node that has a and b as descendants. We have to keep in mind that a node can be a descendant of itself.

So, if the input is like

a = 6, b = 2, then the output will be 4

To solve this, we will follow these steps −

• Define a method solve() this will take root and a, b

• if root is null, then

  • return -1

• if value of root is either a or b, then

  • return value of root

• left := solve(left of root, a, b)

• right := solve(right of root, a, b)

• if left or right is not -1, then

  • return value of root

• return left if left is not same as -1 otherwise right

• from the main method call solve(root)

Let us see the following implementation to get better understanding −

Example

 Live Demo

class TreeNode:
   def __init__(self, val, left=None, right=None):
      self.val = val
      self.left = left
      self.right = right

class Solution:
   def solve(self, root, a, b):
      if not root:
         return -1
      if root.val in (a, b):
         return root.val
      left = self.solve(root.left, a, b)
      right = self.solve(root.right, a, b)
      if -1 not in (left, right):
         return root.val
      return left if left != -1 else right
ob = Solution()
root = TreeNode(3)
root.left = TreeNode(10)
root.right = TreeNode(4)
root.right.left = TreeNode(8)
root.right.right = TreeNode(2)
root.right.left.left = TreeNode(6)
print(ob.solve(root, 6, 2))

Input

root = TreeNode(3)
root.left = TreeNode(10)
root.right = TreeNode(4)
root.right.left = TreeNode(8)
root.right.right = TreeNode(2)
root.right.left.left = TreeNode(6)
6, 2

Output

4