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274. H-Index

Approach 1: Bucket

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  int hIndex(vector<int>& citations) {
    const int n = citations.size();
    int accumulate = 0;
    vector<int> count(n + 1);

    for (const int citation : citations)
      ++count[min(citation, n)];

    // To find the maximum h-index, loop from the back to the front.
    // i := the candidate's h-index
    for (int i = n; i >= 0; --i) {
      accumulate += count[i];
      if (accumulate >= i)
        return i;
    }

    throw;
  }
};

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class Solution {
  public int hIndex(int[] citations) {
    final int n = citations.length;
    int accumulate = 0;
    int[] count = new int[n + 1];

    for (final int citation : citations)
      ++count[Math.min(citation, n)];

    // To find the maximum h-index, loop from the back to the front.
    // i := the candidate's h-index
    for (int i = n; i >= 0; --i) {
      accumulate += count[i];
      if (accumulate >= i)
        return i;
    }

    throw new IllegalArgumentException();
  }
}

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class Solution:
  def hIndex(self, citations: list[int]) -> int:
    n = len(citations)
    accumulate = 0
    count = [0] * (n + 1)

    for citation in citations:
      count[min(citation, n)] += 1

    # To find the maximum h-index, loop from the back to the front.
    # i := the candidate's h-index
    for i, c in reversed(list(enumerate(count))):
      accumulate += c
      if accumulate >= i:
        return i

Approach 2: Sort

  • Time: $O(\texttt{sort})$
  • Space: $O(1)$
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class Solution {
 public:
  int hIndex(vector<int>& citations) {
    const int n = citations.size();

    ranges::sort(citations);

    for (int i = 0; i < n; ++i)
      if (citations[i] >= n - i)
        return n - i;

    return 0;
  }
};

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class Solution {
  public int hIndex(int[] citations) {
    final int n = citations.length;

    Arrays.sort(citations);

    for (int i = 0; i < n; ++i)
      if (citations[i] >= n - i)
        return n - i;

    return 0;
  }
}

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class Solution:
  def hIndex(self, citations: list[int]) -> int:
    n = len(citations)

    citations.sort()

    for i, citation in enumerate(citations):
      if citation >= n - i:
        return n - i

    return 0