Building Boxes LeetCode solution in Java

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class Solution {
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  static final double ONE_THIRD = 1.0d / 3.0d;
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  public int minimumBoxes(int n) {
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    int k = findLargestTetrahedralNotGreaterThan(n);
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    int used = tetrahedral(k);
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    int floor = triangular(k);
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    int unused = (n - used);
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    if (unused == 0) {
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      return floor;
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    }
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    int r = findSmallestTriangularNotLessThan(unused);
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    return (floor + r);
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  }
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  private final int findLargestTetrahedralNotGreaterThan(int te) {
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    int a = (int) Math.ceil(Math.pow(product(6, te), ONE_THIRD));
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    while (tetrahedral(a) > te) {
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      a--;
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    }
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    return a;
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  }
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  private final int findSmallestTriangularNotLessThan(int t) {
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    int a = -1 + (int) Math.floor(Math.sqrt(product(t, 2)));
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    while (triangular(a) < t) {
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      a++;
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    }
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    return a;
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  }
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  private final int tetrahedral(int a) {
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    return (int) ratio(product(a, a + 1, a + 2), 6);
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  }
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  private final int triangular(int a) {
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    return (int) ratio(product(a, a + 1), 2);
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  }
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  private final long product(long... vals) {
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    long product = 1L;
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    for (long val : vals) {
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      product *= val;
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    }
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    return product;
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  }
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  private final long ratio(long a, long b) {
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    return (a / b);
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  }
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}