The code looks scary, but in the end it just comes down to perseverance and carefully tracking your work. Try to figure out what each line does and look up unfamiliar functions. IDLE was very helpful to see what all the list indicing and map functions do. I've attached my understanding of the code, commented, below.
Once you figure out what each line does, the only catch is to keep track of the popped indices in relation with their original position in s. The flag just need to be any string passing the check, so any string of the form
Wh4/___/___0_0_0_0_M3//&&