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

  • 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.

Solution

Think of solving this problem, by using a frequency diagram. For each possible h-value (0 - n), we keep track of papers that have exact h-citations which is in O(n). We iterate the frequency array from right to left, while cumulating the frequencies and break if cumulative >= i, and consequently return i.

class Solution {
    public int hIndex(int[] citations) {
        int n = citations.length;
        int freq[] = new int[n+1]; // frequency for each possible h_value

        for(int citation : citations){
            if (citation >= n){
                freq[n]++; // if it has more than n citations, it goes to h=n.
            }else{
                freq[citation]++;
            }
        }

        int cumulative = 0;
        for(int i=n; i>= 0; --i){
            cumulative += freq[i];

            if(cumulative >= i){
                return i;
            }
        }
        return 0;
    }
}