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1998. GCD Sort of an Array 👍

  • Time: $O(k\log^* k + k\log(\log k) + \texttt{sort}(n))$, where $k = \max(\texttt{nums})$
  • Space: $O(\max(\texttt{nums}) + n)$
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class UnionFind {
 public:
  UnionFind(int n) : id(n), rank(n) {
    iota(id.begin(), id.end(), 0);
  }

  void unionByRank(int u, int v) {
    const int i = find(u);
    const int j = find(v);
    if (i == j)
      return;
    if (rank[i] < rank[j]) {
      id[i] = j;
    } else if (rank[i] > rank[j]) {
      id[j] = i;
    } else {
      id[i] = j;
      ++rank[j];
    }
  }

  int find(int u) {
    return id[u] == u ? u : id[u] = find(id[u]);
  }

 private:
  vector<int> id;
  vector<int> rank;
};

class Solution {
 public:
  bool gcdSort(vector<int>& nums) {
    const int mx = ranges::max(nums);
    const vector<int> minPrimeFactors = sieveEratosthenes(mx + 1);
    UnionFind uf(mx + 1);

    for (const int num : nums)
      for (const int primeFactor : getPrimeFactors(num, minPrimeFactors))
        uf.unionByRank(num, primeFactor);

    vector<int> sortedNums(nums);
    ranges::sort(sortedNums);

    for (int i = 0; i < nums.size(); ++i)
      // Can't swap nums[i] with sortedNums[i].
      if (uf.find(nums[i]) != uf.find(sortedNums[i]))
        return false;

    return true;
  }

 private:
  // Gets the minimum prime factor of i, where 1 < i <= n.
  vector<int> sieveEratosthenes(int n) {
    vector<int> minPrimeFactors(n + 1);
    iota(minPrimeFactors.begin() + 2, minPrimeFactors.end(), 2);
    for (int i = 2; i * i < n; ++i)
      if (minPrimeFactors[i] == i)  // `i` is prime.
        for (int j = i * i; j < n; j += i)
          minPrimeFactors[j] = min(minPrimeFactors[j], i);
    return minPrimeFactors;
  }

  vector<int> getPrimeFactors(int num, const vector<int>& minPrimeFactors) {
    vector<int> primeFactors;
    while (num > 1) {
      const int divisor = minPrimeFactors[num];
      primeFactors.push_back(divisor);
      while (num % divisor == 0)
        num /= divisor;
    }
    return primeFactors;
  }
};

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class UnionFind {
  public UnionFind(int n) {
    id = new int[n];
    rank = new int[n];
    for (int i = 0; i < n; ++i)
      id[i] = i;
  }

  public void unionByRank(int u, int v) {
    final int i = find(u);
    final int j = find(v);
    if (i == j)
      return;
    if (rank[i] < rank[j]) {
      id[i] = j;
    } else if (rank[i] > rank[j]) {
      id[j] = i;
    } else {
      id[i] = j;
      ++rank[j];
    }
  }

  public int find(int u) {
    return id[u] == u ? u : (id[u] = find(id[u]));
  }

  private int[] id;
  private int[] rank;
}

class Solution {
  public boolean gcdSort(int[] nums) {
    final int mx = Arrays.stream(nums).max().getAsInt();
    final int[] minPrimeFactors = sieveEratosthenes(mx + 1);
    UnionFind uf = new UnionFind(mx + 1);

    for (final int num : nums)
      for (final int primeFactor : getPrimeFactors(num, minPrimeFactors))
        uf.unionByRank(num, primeFactor);

    int[] sortedNums = nums.clone();
    Arrays.sort(sortedNums);

    for (int i = 0; i < nums.length; ++i)
      // Can't swap nums[i] with sortedNums[i].
      if (uf.find(nums[i]) != uf.find(sortedNums[i]))
        return false;

    return true;
  }

  // Gets the minimum prime factor of i, where 1 < i <= n.
  private int[] sieveEratosthenes(int n) {
    int[] minPrimeFactors = new int[n + 1];
    for (int i = 2; i <= n; ++i)
      minPrimeFactors[i] = i;
    for (int i = 2; i * i < n; ++i)
      if (minPrimeFactors[i] == i) // `i` is prime.
        for (int j = i * i; j < n; j += i)
          minPrimeFactors[j] = Math.min(minPrimeFactors[j], i);
    return minPrimeFactors;
  }

  private List<Integer> getPrimeFactors(int num, int[] minPrimeFactors) {
    List<Integer> primeFactors = new ArrayList<>();
    while (num > 1) {
      final int divisor = minPrimeFactors[num];
      primeFactors.add(divisor);
      while (num % divisor == 0)
        num /= divisor;
    }
    return primeFactors;
  }
}

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class UnionFind:
  def __init__(self, n: int):
    self.id = list(range(n))
    self.rank = [0] * n

  def unionByRank(self, u: int, v: int) -> None:
    i = self.find(u)
    j = self.find(v)
    if i == j:
      return False
    if self.rank[i] < self.rank[j]:
      self.id[i] = j
    elif self.rank[i] > self.rank[j]:
      self.id[j] = i
    else:
      self.id[i] = j
      self.rank[j] += 1
    return True

  def find(self, u: int) -> int:
    if self.id[u] != u:
      self.id[u] = self.find(self.id[u])
    return self.id[u]


class Solution:
  def gcdSort(self, nums: list[int]) -> bool:
    mx = max(nums)
    minPrimeFactors = self._sieveEratosthenes(mx + 1)
    uf = UnionFind(mx + 1)

    for num in nums:
      for primeFactor in self._getPrimeFactors(num, minPrimeFactors):
        uf.unionByRank(num, primeFactor)

    for a, b in zip(nums, sorted(nums)):
      # Can't swap nums[i] with sortedNums[i].
      if uf.find(a) != uf.find(b):
        return False

    return True

  def _sieveEratosthenes(self, n: int) -> list[int]:
    """Gets the minimum prime factor of i, where 1 < i <= n."""
    minPrimeFactors = [i for i in range(n + 1)]
    for i in range(2, int(n**0.5) + 1):
      if minPrimeFactors[i] == i:  # `i` is prime.
        for j in range(i * i, n, i):
          minPrimeFactors[j] = min(minPrimeFactors[j], i)
    return minPrimeFactors

  def _getPrimeFactors(self, num: int, minPrimeFactors: list[int]) -> list[int]:
    primeFactors = []
    while num > 1:
      divisor = minPrimeFactors[num]
      primeFactors.append(divisor)
      while num % divisor == 0:
        num //= divisor
    return primeFactors