topological order: 1 3 2 5 4 6 7

relaxing in topological order
  v  dist[v]  via
  1        0     0
  3        4     1
  2        6     1
  5       15     2
  4       11     2
  6       19     5
  7       22     6

longest path has weight 22: 1 2 5 6 7
