Fewest swaps to balance brackets

You have a string of only [ and ], with an equal count of each, so a balanced arrangement is always achievable. A single swap exchanges any two characters (not necessarily adjacent).

"[]"       -> 0
"]["       -> 1     # swap the two characters
"]]][[["   -> 2
"][]["     -> 1

Return the minimum number of swaps needed to make the string balanced. Aim for $O(n)$.