In this post, checkpoint traces on World’s Hardest 36 and 200, and a direct comparison with two Strmckr solutions to Worlds 25. Also, a Gordon Fick contribution to the cause.
Here is Strmckr’s first solution, right out of line marking. It’s an almost nice loop AIC bringing the adjacent winks together to remove 2r1c7.
It looks easy, but it’s not, considering what you have to be looking for, to spot it. There are two multiple value cells and two AIC hinges involved. Strmckr’s “singles” collapse is rapid and complete.
At the point in the SysOB where AIC hinges are applied, I have two coloring clusters.
Instead of the pattern analysis and trial, I should have noticed the decisive trap. It’s that both blue and green remove 2r1c7, the same removal as Strmckr’s AIC ANL.
Coloring hastens the collapse as well. The blue/green cluster expands before 6r7c3 wraps green.
The approach towards a BUG explains why 25 can be very hard.
I’ve now released Strmckr’s 4/24/18 comment with his second solution.
It involves a cleverly selected ALS pair. Strmckr takes no credit for that, but instead, ascribes the removals to “transport”, a forum concept whose definition does not respond to my search.
No worries. With the pair of 2’s as a restricted common, 4r9c7, and 6r7c7 qualify as ALS_XZ victims. Shortly after, 6r4c2 and 2r1c7 follow.
But there’s more. Sysudoku’s own ALS_XZ expert Gordon Fick responded to the homework challenge and the ALS Partnering idea with an apt alternative.
Working with the bv 28r1c8, try the possible RC with 8r8c8. The r8 row has extra 8’s, but there is an SE ALS with 2 and a single 8.
Now for the checkpoint basic traces. First the basic rout of Very Hard 80, one of two rated second highest by Sudokubooks. Obviously not the monster.
If you’ve never read a trace, filling in the grid as you go, try it out. It took me several tries to get the trace right, and it is very useful. Just give yourself time to know exactly what each move is, and why it is known to work. Read Sysudoku Traces as necessary.
The second trace then, is for a near monster, World’s Hardest 200.
Here is the line marked grid, with fill strings. If you can solve it, tell me how you got it started. I’m sure sysudokie readers will be interested.
Next week, the World’s Hardest review continues with 200.