Lowest Common Ancestor of a Binary Tree in Python


Suppose we have a binary tree. we have to find the Lowest common ancestor nodes of two given nodes. The LCA of two nodes p and q is actually as the lowest node in tree that has both p and q as decedent. So if the binary tree is like [3,5,1,6,2,0,8,null,null,7,4]. The tree will be like −


Here LCA of 5 and 1 is 3

To solve this, we will follow these steps −

  • If the tree is empty, then return null

  • if p and q both are same as root, then return root

  • left := LCA of left subtree of the root using p and q

  • right := LCA of right subtree of the root using p and q

  • if left and right both are non-zero, then return root

  • return left OR right

Let us see the following implementation to get better understanding −

Example

class Solution(object):
   def lowestCommonAncestor(self, root, p, q):
      """
      :type root: TreeNode
      :type p: TreeNode
      :type q: TreeNode
      :rtype: TreeNode
      """
      if not root:
         return None
      if root == p or root==q:class Solution(object):
   def lowestCommonAncestor(self, root, p, q):
      """
      :type root: TreeNode
      :type p: TreeNode
      :type q: TreeNode
      :rtype: TreeNode
      """
      if not root:
         return None
      if root == p or root==q:
         return root
      left = self.lowestCommonAncestor(root.left, p, q)
         right = self.lowestCommonAncestor(root.right, p, q)
            if right and left:
               return root
               return right or left

Input

[3,5,1,6,2,0,8,null,null,7,4]
5
1

Output

3
raja
Published on 05-Feb-2020 10:18:42
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