Isolation Centers

As a health expert, Vinay is keeping a close watch on the ongoing pandemic of coronavirus disease (COVID-19). He thought of a different situation where there are $26$ types of viruses, named "**a**orona", "**b**orona", "**c**orona", ..., "**z**orona". You are given a string $S$ with length $N$. There are $N$ people (numbered $1$ through $N$) and for each valid $i$, the $i$-th person is infected