  We present an universal way to concentrate an arbitrary $N$-particle
less-entangled $W$ state into a maximally entangled $W$ state with different
parity check gates. It comprises two protocols. The first protocol is based on
the linear optical elements say the partial parity check gate and the second
one uses the quantum nondemolition (QND) to construct the complete parity check
gate. Both of which can achieve the concentration task. These protocols have
several advantages. First, it can obtain a maximally entangled W state only
with the help of some single photons, which greatly reduces the number of
entanglement resources. Second, in the first protocol, only linear optical
elements are required which is feasible with current techniques. Third, in the
second protocol, it can be repeated to perform the concentration step and get a
higher success probability. All these advantages make it be useful in current
quantum communication and computation applications.
