At the first time I met this problem, there were two solution stroke my brain:
1. Directly exchange elements' position. 2. Use C++ STL remove_if() function. Solution 1 seems complex for me because extra space for algorithm is forbidden. After I coded Solution 2, I found how to code Solution 1. We can directly discard zeroes. After the rearrangement, we can "recover" the zeroes in the last n slots in the array.