title: Sort lists in an adjacency list each in increasing order
body: <p>In a directed graph I want an O(n+m) algorithm to sort lists in an adjacency list, so that names of vertices are sorted in increasing order in each list. 
  The only one I can think of is to perform an insertion sort, on each of the list, but this definitely does not run in O(n+m). Can someone help me with this?
  Thanks</p>