H-Index -- Leetcode

Question:

Given an array of citations (each citation is a non-negative integer) of a researcher, write a function to compute the researcher's h-index.

According to the definition of h-index on Wikipedia: "A scientist has index h if h of his/her N papers have at least h citations each, and the other N − h papers have no more than h citations each."

For example, given citations = [3, 0, 6, 1, 5], which means the researcher has 5 papers in total and each of them had received 3, 0, 6, 1, 5 citations respectively. Since the researcher has 3 papers with at least 3 citations each and the remaining two with no more than 3 citations each, his h-index is 3.

Note: If there are several possible values for h, the maximum one is taken as the h-index.

Hint:

1. An easy approach is to sort the array first.
2. What are the possible values of h-index?
3. A faster approach is to use extra space.

Answer:

Time: O(n), Space:O(n), Using bucket sort or hashmap, faster than sort method! 

public int hIndex(int[] citations) {
        int len = citations.length;
        HashMap<Integer, Integer> hmap = new HashMap<Integer, Integer>();
        for(int i=0; i<len; ++i){
            int val = citations[i];
            if(val >= 1 && val <= len){
                if(!hmap.containsKey(val)){
                    hmap.put(val, 1);
                }else{
                    hmap.put(val, hmap.get(val) + 1);
                }
            }else if(val > len){
                if(!hmap.containsKey(len)){
                    hmap.put(len, 1);
                }else{
                    hmap.put(len, hmap.get(len) + 1);
                }
            }
        }
        int count = 0;
        for(int i=len; i>=1; --i){
            if(hmap.containsKey(i)){
                count += hmap.get(i);
            }
            if(count >= i){
                return i;
            }
        }
        return 0;
    }