[leetcode] H-Index (Hash Table)

题目：

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.

分析：借助一个哈希表(辅助数组)，实现时间复杂度为O(N). 构造好辅助数组后，从数组后面开始遍历，如果paper的总数>=引用数，返回该引用数即可。

Java代码如下：

    public int hIndex(int[] citations) {
        int[] tmp = new int[citations.length + 1];
        for (int i = 0; i < citations.length; i++) {
            if (citations[i] >= citations.length) {
                tmp[citations.length]++;
            } else {
                tmp[citations[i]]++;
            }
        }
        int sum = 0;
        for (int i = tmp.length - 1; i >= 0; i--) {
            sum += tmp[i];
            if (sum >= i) {
                return i;
            }
        }
        return 0;
    }