1963 - Minimum Number of Swaps to Make the String Balanced\.

Medium

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

• <code>2 <= n <= 10⁶</code>

• 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.