The Undisappearance

You are given an array $A$ of length $N$ containing only the integers $1, 2, $ and $3$. It is guaranteed that each of $1, 2, $ and $3$ appear at least once in $A$. Consider the following operation on $A$: 1. First, choose two integers $L$ and $R$ such that $1 \leq L \leq R \leq N$. 2. Then, for each index $i$ from $L$ to $R$, - If $A_i = 1$, replace it with $2$. - If $A_i = 2$, replace