LeetCode-in-Java

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
• 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.

Solution

public 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;
    }
}

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