[sam_in_nyc]

What got me at first was reading the solution and thinking: "Oh, so they're allowed to label the boxes beforehand? Lame." I assumed by "label" the solutions meant "physically write on the box."

It took awhile to sink in that the prisoners could use their own memory, instead. Randomly line up, and have each prisoner memorize all prisoners' numbers in line. Then, agree on the numbering structure of the boxes in the room (eg: left to right = 0 to 100).

This fails if the warden is allowed to shuffle the boxes before each prisoner selects (not likely). It also fails if the room were rotationally symmetric, and each prisoner was brought in from one of two entrances. This way he would be unable to identify the "left" end of the line of boxes. (Much more likely, were I warden).