What is Nim Game and how it is played?
It is a game in which two players play optimally turn by turn .

In each turn a player can choose only one pile and remove any number of stones (at least one) from that pile.

The person who is unable to make a move looses , hence the player who makes the last move wins.

determine which player wins the game.
How to decide who will win?
To decide who will win the initial configuration of the piles matter.

If the xor of all elements of the array is nonzero , then person starting the game will win.

If the xor of all elements of the array is zero , then other person will win.

Why it works ?

if the xor if all elements is zero then if we make any moves , the xor of all elements will definitely become non zero .
Why?
If the xor of all elements is non zero , then we can either make the xor nonzero again , or make the xor zero.

There definitely exists atleast one way to make xor zero again.
Why?
How to reach the Conclusion 
Conclusion 
Hence if xor is zero B Wins , else A wins