275. H-Index II

Follow up for H-Index: What if the citations array is sorted in ascending order? Could you optimize your algorithm?



Solution:

The idea is to use binary search to find the first occurrence satisfy:

citations[index] >= citations.length - index.

This condition means there are citations[index] papers have at least citations[index] citations.

The time complexity is O(logn) and the space complexity is O(n).



Code:

public class Solution {
    public int hIndex(int[] citations) {
        if (citations == null || citations.length == 0) {
            return 0;
        }
        // find first occurance that citations[index] >= citations.length - index
        int n = citations.length;
        int start = 0;
        int end = n - 1;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (citations[mid] >= n - mid) {
                end = mid;
            }
            else {
                start = mid;
            }
        }
        if (citations[start] >= n - start) {
            return n - start;
        }
        if (citations[end] >= n - end) {
            return n - end;
        }
        return 0;
    }
}