It is a common tactic on the far right (the holocaust, the supposed rape epidemic in Sweden) and left (9/11, the holocaust too!) to simply declare politically troubling material 'false' out of hand.
Your examples seem a bit mixed up to me.
Holocaust - Far Right claims didn't happen. Often they take a synecdoche approach where eg Auschwitz-Birkenau is made to stand in for the entire Holocaust, and attack a straw man. But I've never seen the Far Left deny the Nazi Holocaust. Islamists do, but they are only allied with the Far Left, they're not Far Left themselves - indeed they get a lot of inspiration from Hitler & co.
Surely the Far Left view is that all whites/Germans/conservatives etc are guilty of the Holocaust, could do it again at any time, and must be punished/repressed accordingly?
Rape epidemic in Sweden (Sweden, Rape Capital of Europe) - hmm, well the reports are true, but there is an argument that the Swedish definition of rape is very broad and may well skew the figures.
9/11 - claims that 9/11 was a CIA/Mossad job seem to derive mostly from the Far Right, though quite ordinary Arabs I've met also seem to believe this (one got rather annoyed when she'd said: "
Arabs could never do something like that!" and I replied "What, you don't think they have the competence?") :p
Personally I've not seen the secular Far Left ever claim this, but I can believe it - what do they say? CIA? Cheney conspiring war-for-oil?