Letter Combinations of a Phone Number LeetCode solution in Java

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class Solution {
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  String[] num = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
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  public List<String> letterCombinations(String digits) {
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    List<String> ll = new ArrayList<>();
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    StringBuilder sb = new StringBuilder();
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    if (digits.length() != 0) {
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      combination(digits.toCharArray(), ll, sb, 0);
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    }
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    return ll;
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  }
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  public void combination(char[] digits, List<String> ll, StringBuilder sb, int idx) {
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    if (sb.length() == digits.length) {
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      ll.add(sb.toString());
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      return;
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    }
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    String grp = num[digits[idx] - 48];
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    for (int i = 0; i < grp.length(); i++) {
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      sb.append(grp.charAt(i));
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      combination(digits, ll, sb, idx + 1);
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      sb.deleteCharAt(sb.length() - 1);
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    }
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  }
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}