H-Index

public class Solution {
    public int hIndex(int[] citations) {
        if(citations == null || citations.length == 0)
            return 0;

        Arrays.sort(citations); // 0 1 3 5 6
        for(int i = 0; i < citations.length; i++){
            int index = citations.length - i;
            if(citations[i] >= index)
                return index;
        }
        return 0;
    }
    
}