274. H Index

Problem¶

Given an array of integers citations where citations[i] is the number of citations a researcher received for their ith paper, return the researcher’s h-index.

According to the definition of h-index on Wikipedia: The h-index is defined as the maximum value of h such that the given researcher has published at least h papers that have each been cited at least h times.

Example 1:

Input: citations = [3,0,6,1,5]
Output: 3
Explanation: [3,0,6,1,5] means the researcher has 5 papers in total and each of them had received 3, 0, 6, 1, 5 citations respectively.
Since the researcher has 3 papers with at least 3 citations each and the remaining two with no more than 3 citations each, their h-index is 3.

Example 2:

Input: citations = [1,3,1]
Output: 1

Constraints:

• n == citations.length
• 1 <= n <= 5000
• 0 <= citations[i] <= 1000

Solve¶

O(n log n) because of sort. Count sort could make this better
- Using binary search knowing that “at least h papers that have each been cited at least h times.” Which mean, finding the starting point where citations[m] >= n-m

class Solution:
    def hIndex(self, citations: List[int]) -> int:
        citations.sort()
        n = len(citations)
        l = -1
        r = n
        while True:
            m = (l + r) // 2
            if l == m:
                break
            if citations[m] >= n-m:
                r = m
            else:
                l = m
        return n-r