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• # For Solution

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`.