code: private void mergeForceCollapse() {
          while (stackSize > 1) {
              int n = stackSize - 2;
              if (n > 0 && runLen[n - 1] < runLen[n + 1])
                  n--;
              mergeAt(n);
          }
      }
docs: Merges all runs on the stack until only one remains.  This method is
  called once, to complete the sort.