275. H-Index II

The idea is to search for the first index from the sorted array so that :

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

And return (length - index) as the result.


99.93 %
class Solution {
    public int hIndex(int[] citations) {
        if (citations == null || citations.length == 0) {
            return 0;
        }
        // [1, 2, 3, 3]
        int start = 0;
        int end = citations.length - 1;
        int len = citations.length;
        while (start < end) {
            int mid = start + (end - start) / 2;
            // [mid, end] -> all have at least citations[mid] citations each
            if (citations[mid] >= len - mid) {
                end = mid;
            } else if (citations[mid] < len - mid + 1) {
                start = mid + 1;
            }
        }
        return citations[start] >= len - start ? len - start : 0;
    }

}