Balance Zeroes and Ones

You are given a string $S$, each of whose characters is either `'0'`, `'1'`, or `'?'`. The *badness* of a binary string is defined to be the (absolute) difference between the number of $1$s and the number of $0$s present in it. For example, $001$ has badness $|1-2| = 1$ and $11111$ has badness $|5-0| = 5$. Your task is to replace every `'?'` present in $S$ with either `'0'` or `'1'`, such that t