Minimum Remove to Make Valid Parentheses LeetCode solution in Java

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class Solution {
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  public String minRemoveToMakeValid(String s) {
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    Stack<Integer> stack = new Stack<>();
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    for (int i = 0; i < s.length(); i++) {
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      char ch = s.charAt(i);
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      if (Character.isAlphabetic(ch)) continue;
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      if (ch == '(') stack.push(i);
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      else {
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        if (!stack.isEmpty() && s.charAt(stack.peek()) == '(') stack.pop();
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        else stack.push(i);
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      }
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    }
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    // if(stack.size() == 0) return "";
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    StringBuilder result = new StringBuilder();
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    HashSet<Integer> set = new HashSet<>(stack);
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    for (int i = 0; i < s.length(); i++) if (!set.contains(i)) result.append(s.charAt(i));
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    return result.toString();
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  }
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}