1963. Minimum Number of Swaps to Make the String Balanced

Problem

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 <= 10^6

• 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.

Solution (Java)

class Solution {
    public int minSwaps(String s) {
        int openCount = 0;
        int swap = 0;
        for (char c : s.toCharArray()) {
            if (c == '[') {
                openCount++;
            } else {
                if (openCount == 0) {
                    swap++;
                    openCount++;
                } else {
                    openCount--;
                }
            }
        }
        return swap;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).