函数f(x)=_{$\left\{\begin{array}{c}x,x\in P\\ -x,x\in M\end{array}\right.$}其中P，M为实数集R的两个非空子集，又规定f(P)={y|y=f(x),x∈P}，f(M)={y|y=f(x),x∈M}.给出下列四个判断：①若P∩M=_{$\varnothing$}，则f(P)∩f(M)=_{$\varnothing$}；  ②若P∩M≠_{$\varnothing$}，则f(P)∩f(M) ≠_{$\varnothing$}；③若P∪M=R，则f(P)∪f(M)=R； ④若P∪M≠R，则f(P) ∪f(M)≠R其中正确判断的有（ ） 

You won't want to visit any other mountain if you have seen Wu yue, but you won't wish to see even Wu yue _______returning from Mount Huangshan.