Hello All.

Please watch this Data Structure :

order_of_key

Today I found this STL from this solution ( 15th Line from the Last Line ) : http://codeforces.net/contest/658/submission/16995396
of Problem B [ VK CUP DIV2 ] .

Later when I have goggled about it I found a Blog written by adamant about this Topic . Here is the Link : http://codeforces.net/blog/entry/11080

But I am failed to understand the actual Purpose & the vast application of this STL . Undoubtedly its a Great Blog & Technique . And with due respect I Thank him .

Now here is My Queries :

What STLs are used ? Why they are used ? How they should be used ? In which Problems I can use this DS ? Are there any special requirments to implement those ?

Thanks in Advance . It would be great help .