Fancy Sequence LeetCode solution in Java

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class Fancy {
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  private ArrayList<Long> lst;
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  private ArrayList<Long> add;
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  private ArrayList<Long> mult;
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  private final long MOD = 1000000007;
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  public Fancy() {
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    lst = new ArrayList<>();
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    add = new ArrayList<>();
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    mult = new ArrayList<>();
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    add.add(0L);
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    mult.add(1L);
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  }
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  public void append(int val) {
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    lst.add((long) val);
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    int l = add.size();
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    add.add(add.get(l - 1));
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    mult.add(mult.get(l - 1));
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  }
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  public void addAll(int inc) {
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    int l = add.size();
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    add.set(l - 1, add.get(l - 1) + inc);
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  }
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  public void multAll(int m) {
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    int l = add.size();
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    add.set(l - 1, (add.get(l - 1) * m) % MOD);
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    mult.set(l - 1, (mult.get(l - 1) * m) % MOD);
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  }
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  public int getIndex(int idx) {
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    if (idx >= lst.size()) return -1;
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    int l = add.size();
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    long m = (mult.get(l - 1) * inverse(mult.get(idx))) % MOD;
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    long a = (add.get(l - 1) - (add.get(idx) * m) % MOD + MOD) % MOD;
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    return (int) (((lst.get(idx) * m) % MOD + a) % MOD);
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  }
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  long inverse(long a) {
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    return pow(a, MOD - 2);
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  }
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  long pow(long a, long n) {
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    if (n == 0) return 1;
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    if (n % 2 == 0) {
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      long t = pow(a, n / 2);
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      return (t * t) % MOD;
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    } else {
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      return (pow(a, n - 1) * a) % MOD;
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    }
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  }
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}