100 Doors Puzzle || Hard Puzzle for Genius minds

There are 100 doors in a row. They all are initially closed.
A person walks through all the doors 100 times and every time he toggles the state of speific doors. that means he changes
the state of the door,. if the door is closed, he will open it.. and if its opened then he will close it.
In each walk he selects specific doors to toggle, in the following manner.

In his first walk, he toggles all the doors in multiple of 1.. that means he toggles all the 100 doors.
In the second walk, he toggles the doors in multiple of 2.
In the third walk, he toggles the doors in multiple of 3. that means, he will change the state of door no.3, 6, 9, 12... and
so on.
In the fourth walk he toggles doors in multiple of 4.

This way he walks for 100 times.

The puzzle is : What door numbers will remain opened after 100 walks.
(Remeber that initially all doors are in closed state)

This video explains this puzzle the best way. You will certainly learn the optimal way and the best approach of solving difficult puzzles.

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