Lowest Common Ancestor of a Binary Search Tree Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.
According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p
and q
as the lowest node in T
that has both p
and q
as descendants (where we allow a node to be a descendant of itself).”
Example 1:



Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8 Output: 6 Explanation: The LCA of nodes 2 and 8 is 6.
Example 2:



Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4 Output: 2 Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.
Example 3:
Input: root = [2,1], p = 2, q = 1 Output: 2
Constraints:
- The number of nodes in the tree is in the range
[2, 105]
. -109 <= Node.val <= 109
- All
Node.val
are unique. p != q
p
andq
will exist in the BST.
Lowest Common Ancestor of a Binary Search Tree Solutions
✅Time: O(h)
✅Space: O(h)
C++
class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if (root->val > max(p->val, q->val))
return lowestCommonAncestor(root->left, p, q);
if (root->val < min(p->val, q->val))
return lowestCommonAncestor(root->right, p, q);
return root;
}
};
Java
class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (root.val > Math.max(p.val, q.val))
return lowestCommonAncestor(root.left, p, q);
if (root.val < Math.min(p.val, q.val))
return lowestCommonAncestor(root.right, p, q);
return root;
}
}
Python
class Solution:
def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
if root.val > max(p.val, q.val):
return self.lowestCommonAncestor(root.left, p, q)
if root.val < min(p.val, q.val):
return self.lowestCommonAncestor(root.right, p, q)
return root
Watch Tutorial
Checkout more Solutions here