Minimum Add to Make Parentheses Valid LeetCode solution in Java

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class Solution {
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  public int minAddToMakeValid(String s) {
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    int open = 0;
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    int extra = 0;
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    for (int i = 0; i < s.length(); i++) {
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      if (s.charAt(i) == '(') {
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        open++;
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      } else {
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        if (open == 0) {
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          extra++;
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        } else {
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          open--;
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        }
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      }
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    }
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    return open + extra;
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  }
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}