200. H-Index

Sort citations from most-cited to least. After sorting, the paper at 0-based index i is the (i+1)th most cited. The h-index is the largest i+1 for which that paper still has at least i+1 citations.

Idea

Sort the citation counts in descending order. Now scanning left to right, at position i (0-based) we have i+1 papers each cited at least citations[i] times. The condition for an h-index of i+1 is exactly citations[i] >= i + 1. As we move right the paper count grows but the citation floor drops, so the h-index is the last position where the condition still holds.

Algorithm

1. Sort citations descending.
2. Walk i from 0: while citations[i] >= i + 1, set h = i + 1. Stop at the first failure.
3. Return h.

Worked example: [3,0,6,1,5]

Sorted desc: [6,5,3,1,0].

h-index = 3 (3 papers with ≥ 3 citations).

Complexity

• Time: O(n log n) for the sort.
• Space: O(1) beyond the sort.

import java.util.Arrays;

class Solution {
    public int hIndex(int[] citations) {
        Arrays.sort(citations);
        int n = citations.length;
        int h = 0;
        for (int i = 0; i < n; i++) {
            int count = n - i;
            if (citations[i] >= count) {
                h = count;
                break;
            }
        }
        return h;
    }
}