Lexicographically Smallest String After Applying Operations LeetCode solution in Java

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class Solution {
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  private String result;
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  public String findLexSmallestString(String s, int a, int b) {
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    result = "z";
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    HashSet<String> set = new HashSet<>();
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    dfs(s, a, b, set);
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    return result;
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  }
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  private void dfs(String s, int a, int b, HashSet<String> set) {
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    if (set.contains(s)) return;
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    set.add(s);
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    String s1, s2;
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    s1 = addA(s, a);
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    s2 = rotateB(s, b);
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    dfs(s1, a, b, set);
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    dfs(s2, a, b, set);
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  }
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  private String addA(String s, int a) {
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    char c[] = s.toCharArray();
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    int i, temp;
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    for (i = 1; i < s.length(); i += 2) {
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      temp = c[i] - '0';
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      temp += a;
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      temp = temp % 10;
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      c[i] = (char) (temp + '0');
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    }
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    s = new String(c);
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    if (result.compareTo(s) > 0) result = s;
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    return s;
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  }
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  private String rotateB(String s, int b) {
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    if (b < 0) b += s.length();
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    b = b % s.length();
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    b = s.length() - b;
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    s = s.substring(b) + s.substring(0, b);
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    if (result.compareTo(s) > 0) result = s;
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    return s;
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  }
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}