Suppose we have a binary search 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 [6, 2, 8, 0, 4, 7, 9, null, null, 3, 5]. The tree will be like −
Here LCA of 2 and 8 is 6
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
Example
Let us see the following implementation to get better understanding −
class TreeNode: def __init__(self, data, left = None, right = None): self.data = data self.left = left self.right = right class Solution(): def lowestCommonAncestor(self, root, p, q): if not root: return None if p == root or q==root: return root left = self.lowestCommonAncestor(root.left, p, q) right = self.lowestCommonAncestor(root.right, p, q) if left and right: return root return left or right def insert(temp,data): que = [] que.append(temp) while (len(que)): temp = que[0] que.pop(0) if (not temp.left): temp.left = TreeNode(data) break else: que.append(temp.left) if (not temp.right): temp.right = TreeNode(data) 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 search_node(root, element): if (root == None): return None if (root.data == element): return root res1 = search_node(root.left, element) if res1: return res1 res2 = search_node(root.right, element) return res2 root = make_tree([6,2,8,0,4,7,9,None,None,3,5]) ob1 = Solution() op = ob1.lowestCommonAncestor(root, search_node(root, 2), search_node(root, 8)) print(op.data)
Input
[6,2,8,0,4,7,9,null,null,3,5] 2 8
Output
6