H-Index

Description

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

Code

用 counting sort 或是其他任何 non-comparison sort 來加速。

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int MAX = *max_element(citations.begin(), citations.end());
        int n = citations.size();
        vector<int> count(MAX + 1, 0);
        vector<int> sorted(n, 0); 
 
        for(auto c: citations) {
            count[c]++;
        }
 
        for(int i = 1; i < count.size(); i++) {
            count[i] += count[i - 1];
        }
 
        for(int i = n - 1; i >= 0; i--) {
            sorted[count[citations[i]] - 1] = citations[i];
            count[citations[i]]--;
        }
        
        for(int i = 0; i < n; i++) {
            if(sorted[i] >= n - i) return n - i;
        }
 
        return 0;
    }
};

Source

• H-Index - LeetCode