Buddy Strings LeetCode solution in Java

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class Solution {
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  public boolean buddyStrings(String s, String goal) {
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    char a = '\u0000', b = '\u0000';
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    char c = '\u0000', d = '\u0000';
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    int lenS = s.length();
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    int lenGoal = goal.length();
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    boolean flag = true;
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    HashSet<Character> hset = new HashSet<>();
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    if (lenS != lenGoal) return false;
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    if (s.equals(goal)) {
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      for (int i = 0; i < lenS; i++) {
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        if (!hset.contains(s.charAt(i))) {
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          hset.add(s.charAt(i));
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        } else return true;
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      }
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      return false;
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    } else {
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      for (int i = 0; i < lenS; i++) {
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        if (s.charAt(i) == goal.charAt(i)) {
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          continue;
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        }
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        if (a == '\u0000') {
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          a = s.charAt(i);
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          c = goal.charAt(i);
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          continue;
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        }
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        if (b == '\u0000') {
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          b = s.charAt(i);
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          d = goal.charAt(i);
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          continue;
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        }
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        return false;
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      }
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      if (a == d && c == b && a != '\u0000') return true;
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      return false;
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    }
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  }
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}