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 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 class Solution(object): def lowestCommonAncestor(self, root, p, q): if not root: return None if root.data == p or root.data ==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 ob1 = Solution() tree = make_tree([3,5,1,6,2,0,8,None,None,7,4]) print(ob1.lowestCommonAncestor(tree, 5, 1).data)
Input
[3,5,1,6,2,0,8,null,null,7,4] 5 1
Output
3