Complexity Analysis

1 . swap between set , vector , multiset is constant complexity . This is just pointer swap . 
     https://www.geeksforgeeks.org/multisetswap-c-stl/
     https://stackoverflow.com/questions/40948838/complexity-of-swapping-two-rows-in-vector-and-array-using-swap-function
     https://www.geeksforgeeks.org/setswap-c-stl/