Intuition

My first thought was to sort the array, and return the first element where count < citations[i]. However this proved to be wrong, since this would return the first element, and after sorting the array we want to return the last element where count < citations[i] is True.

Approach

Sort the array, then iterate over it storing the count where count < citations[i].

Complexity

• Time complexity: O(n log n) since we have the sort function

• Space complexity: O(1)

Code

class Solution(object):
    def hIndex(self, citations):
        """
        :type citations: List[int]
        :rtype: int
        """
        citations = sorted(citations, reverse=True)
        hIndex = 0
        for i in range(len(citations)):
            if (i+1) <= citations[i]:
                hIndex = i+1
        return hIndex