# 1963. Minimum Number of Swaps to Make the String Balanced

<https://leetcode.com/problems/minimum-number-of-swaps-to-make-the-string-balanced>

## Description

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`.

## ac

```java
```