public class Solution {
    public int HIndex(int[] citations) 
    {
        if(citations.Length == 0){
    		return 0;
    	}
    	
    	var sum = 0;
    	var j = 0;
    	for(var i = citations.Length - 1;i >= 0; i--){
    		if(j < citations[i]){
    			sum ++;
    		}
    		j++;
    	}
    	
    	return sum;
    }
}