What is bitset<1000005> b; used in the video but not explained clearly.?

#1

What is bitset<1000005> b; used in the video but not explained clearly.?

#2

bitset is a data structure which is space efficient , instead of taking 4 bytes of memory for integer it only takes 1 bit, it only takes two values 0 and 1.
so bitset<10000005> means creating a bitset array which will contain 0/1.
access elements using b[0] notation same as array .