Blocks in a String

Define a **block** of a binary string to be a maximal substring consisting of the same character. For example, the blocks of $\text{1100100}$ are $\text{11}$, $\text{00}$, $\text{1}$ and $\text{00}$. You are given a string $S$, of size $N$, consisting of characters $0$, $1$ or $?$. It is guaranteed that $S$ does not begin or end with a $?$, i.e. $S_1 \ne ?$ and $S_N \ne ?$. Your job is to replac