Evaluate the Bracket Pairs of a String LeetCode solution in Java

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class Solution {
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  public String evaluate(String s, List<List<String>> knowledge) {
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    Map<String, String> map = new HashMap<>();
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    for (List<String> ele : knowledge) {
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      map.put(ele.get(0), ele.get(1));
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    }
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    StringBuilder sb = new StringBuilder();
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    int b_start = -1;
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    for (int i = 0; i < s.length(); i++) {
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      if (s.charAt(i) == '(') {
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        b_start = i;
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      } else if (s.charAt(i) == ')') {
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        String key = s.substring(b_start + 1, i);
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        sb.append(map.getOrDefault(key, "?"));
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        b_start = -1;
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      } else {
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        if (b_start == -1) {
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          sb.append(s.charAt(i));
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        }
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      }
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    }
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    return sb.toString();
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  }
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}