h-index

问题描述：

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

思路：将数组进行倒序排列，下面的代码是自己写的快排，也可以呀标准库中的排序函数，然后从0开始遍历，遍历到i时，比较citations[i]与i的大小，如果citations[i] > i,由于数组是倒序的，则可以确定有数组中有i+1个数字大于i+1，然后继续遍历，否则i++，然后返回i。

class Solution {
public:
    static int hIndex(vector<int>& citations) {
        if (citations.empty())
            return 0;
        int len = citations.size();
        qSort(citations, 0, len - 1);
        int i = 0;
        for (; i < len && citations[i] > i; i++);
        return i;
    }
private:
    static int partition(vector<int>& list, int low, int high) {
        int pivotkey = list[low];
        while (low < high) {
            while (low < high && pivotkey >= list[high]) high--;
            list[low] = list[high];
            while (low < high&&pivotkey <= list[low]) low++;
            list[high] = list[low];
        }
        list[low] = pivotkey;
        return low;
    }

    static void qSort(vector<int>& list, int low, int high) {
        if (list.empty())
            return;
        if (low < high) {
            int part = partition(list, low, high);
            qSort(list, low, part - 1);
            qSort(list, part + 1, high);
        }
    }
};