meaning of accuracy header in practice problems section

I know that while solving practice problems we can sort them in order of successful submissions or that of accuracy.
I am confused about meaning of accuracy here.
It may mean 2 things

  1. percentage of correct submissions out of all submissions
  2. percentage of efficiency (say how tightly bound successful solutions are like if 5 solutions are correct and 3 out of these run in 0 time so maybe accuracy is (3/5)*100 )

Please clarify.
Also, please suggest weather we should sort by successful submissions or by accuracy