Minimum Number of Swaps to Make the String Balanced LeetCode solution in Java

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class Solution {
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  public int minSwaps(String s) {
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    // remove the balanced part from the given string
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    Stack<Character> stack = new Stack<>();
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    for (char ch : s.toCharArray()) {
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      if (ch == '[') stack.push(ch);
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      else {
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        if (!stack.isEmpty() && stack.peek() == '[') stack.pop();
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        else stack.push(ch);
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      }
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    }
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    int unb = stack.size() / 2; // # of open or close bracket
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    return (unb + 1) / 2;
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  }
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}