Solution
= Solution
Take an <integer> pivot $0\leq p\leq N+1$. There are $p$ possible values in $[0,p)$ but only $k$ occurrences of such values in the whole <array>. When $k<p$, the <pigeonhole principle> gives
$$
\boxed{\text{Some value in }[0,p)\text{ is missing.}}
$$
Repeated values do not invalidate the argument: they can only reduce the number of distinct values represented by those $k$ occurrences. The other side might also contain a missing value.