/// <summary>
/// Find the common ancestor to two nodes. The ancestor of the root node is itself.(!)
/// </summary>
/// <param name="root"></param>
/// <param name="n1"></param>
/// <param name="n2"></param>
/// <returns></returns>
public static lcaResult lca_func(TreeNode root, TreeNode n1, TreeNode n2)
{
if (root == null)
{
return(new lcaResult(0, null));
}
// recursively investigate left subtree
lcaResult leftResult = lca_func(root.left, n1, n2);
if (leftResult.nodesFound == 2)
{
return(leftResult);
}
// recursively investigate right subtree
lcaResult rightResult = lca_func(root.right, n1, n2);
if (rightResult.nodesFound == 2)
{
return(rightResult);
}
// if we got to this point, then:
// - the common ancestor is not in the left subtree, otherwise nodesFound == 2 would have been true earlier
// - ditto
// if one of the nodes is in the left subtree, and the other is in the right subtree, then the current root
// node is the common ancestor we're looking for.
int nodesFound = leftResult.nodesFound + rightResult.nodesFound;
if (root == n1)
{
nodesFound++;
}
if (root == n2)
{
nodesFound++;
}
if (nodesFound == 2)
{
return(new lcaResult(2, root));
}
return(new lcaResult(nodesFound, null));
}
public static TreeNode lca(TreeNode root, TreeNode n1, TreeNode n2)
{
// special cases:
// root == n1, and/or root == n2
if (root == n1 || n1 == n2)
{
return(root);
}
if (root == null)
{
return(null);
}
lcaResult lcaData = lca_func(root, n1, n2);
return(lcaData.ancestor);
}