prove that l={w ∈ {0, 1}*: n0(w) ≠ n1(w)} is a non regular language?
There are two easy proofs that I will let you work out on your own. The first uses the fact that regular languages are closed under complement, the other uses the Myhill–Nerode criterion.

You can also prove that this language is not regular using the pumping lemma, but the proof is a bit tricky. Let $p$ be the pumping length, and consider the word $w = 0^p1^{p!+p} \in L$. The pumping lemma gives us some decomposition $w = xyz$ with $y = 0^i$ for $1 \leq i \leq p$. Then $xy^{1+p!/i}z = 0^{p!+p} 1^{p!+p} \notin L$.