Minimum Number of Swaps to Make the String Balanced

You are given a **0-indexed** string `s` of **even** length `n`. The string consists of **exactly** `n / 2` opening brackets `'['` and `n / 2` closing brackets `']'`. A string is called **balanced** if and only if: - It is the empty string, or - It can be written as `AB`, where both `A` and `B` are **balanced** strings, or - It can be written as `[C]`, where `C` is a **balanced** string. You may swap the brackets at **any** two indices **any** number of times. Return _the **minimum** number of swaps to make _`s` _**balanced**_.   **Example 1:** **Input:** s = "][][" **Output:** 1 **Explanation:** You can make the string balanced by swapping index 0 with index 3. The resulting string is "[[]]". ``` **Example 2:** **Input:** s = "]]][[[" **Output:** 2 **Explanation:** You can do the following to make the string balanced: - Swap index 0 with index 4. s = "[]][][". - Swap index 1 with index 5. s = "[[][]]". The resulting string is "[[][]]". ``` **Example 3:** **Input:** s = "[]" **Output:** 0 **Explanation:** The string is already balanced. ```   **Constraints:** - `n == s.length` - `2 <= n <= 106` - `n` is even. - `s[i]` is either `'[' `or `']'`. - The number of opening brackets `'['` equals `n / 2`, and the number of closing brackets `']'` equals `n / 2`.