We were discussing document libraries and file synchronization service. This service provide incremental synchronization between two file system locations based on change detection which is a process that evaluates changes from last synchronization.
It stores metadata about the synchronization which describes where and when the item was changed, giving a snapshot of every file and folder in the replica. For files, the comparison is done on the file size, file times, file attributes, file name and optionally a hash of the file contents. For folders, the comparison is done on folder attributes and folder names.
Since change detection evaluates all files, a large number of files in the replica may degrade performance.
The file synchronization provider supports extensive progress reporting during the synchronization operation. This can be visualized through the User Interface as a progress bar. This information is reported to the application via events in the managed code or callbacks in the unmanaged code.
The preview mode displays what changes would happen during synchronization. The changes are not committed but the progress notifications are still sent out.
If the users modify different file system replicas, then they may get out of sync and a process of  conflict resolution is performed. The conflict resolution is deterministic. It does not matter which replica initiates the conflict resolution. In all cases, it avoids data loss. The policy for conflict resolution is usually the same. If an existing file is modified independently on two replicas, the file with the last write wins the selection.If an existing file is modified and the same file or folder is deleted in another replica, the deleted will be resurrected. The override for the delete is also the option used when a file is created in a folder which is deleted in another replica. If there are name collisions on files or folders when say one of the folders is renamed, the user will end up with a single folder and the contents merged. The name collision conflicts may happen in other cases also.
If two files are created independently on two replicas with the same name, then there will certainly be a file with that name but with the contents from the side that had the most recent update.
If an existing file is renamed with the name colliding with that of a new file created,then both files are kept and one of them is renamed because the user intended to keep both.
#codingexercise
Given two substrings find if a first string is a subsequence of another
bool IsSubsequence(string one, int len1, string two, int len2)
{
 assert (len1 <= one.Length && len2 <= two.Length);
 if (len1 == 0) return true;
 if (len2 == 0) return false;
 if (one[len1-1] == two[len2-1])
    return IsSubsequence(one, len1-1, two, len2-1);
 return IsSubsequence(one, len1, two, len2-1);
}
/*
AH, DAGH => True
DAGH, AH => False
ABC, ABC => True

*/
int GetSecondLastInBST(Node root)
{
var inorder = new List<int>();
InorderTraversal(root, ref inorder);
if (inorder.Count-2 < 0) throw new Exception("Invalid");
return inorder[inorder.Count-2];
}
// The above can be modified to store only last two elements