# Help me to find least common ancestor in a BST.

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I have tried bruteforce method but it's complexity is of O(n^2) , i am interested in O(n) or O(nlon n)

for the tree given above common ancestor of node 1 and 7 is 4. I am much interested in function not the whole program,function shoud take three argument as root of the tree, node1 and node 2

posted Jun 16, 2016
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A binary tree is given and two nodes of this tree is given, we have to find out the algorithm/code for lowest common ancestor of the two given nodes. Common ancestor are the nodes which can be reached from two nodes by following the parent links. Lowest common ancestor is the common ancestor which has the highest depth.

``````                    10
/  \
7     15
/ \    / \
6   8  12  18
/     \
5       9
(Given Binary tree)
``````
+1 vote

for example in above tree ancestor of 7 is : 7 3 1

``````      40
/\
20 60
/\  \
10 30  80
/   /\
25  70 90
\
75

longest path
25 30 20 40 60 80 70 75