Leetcode编程练习：H指数

题目原文：（id=274)

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.

H指数的定义有些别扭，也找不到其它比上文更好的解释，总之要找到一个数列的H指数的值，因为我们需要知道数列中比某个数大的元素的确切个数，最简单的做法是先对数列中元素进行排序，然后假定每一个元素的值（或大于等于它元素个数）都可能是H指数，逐个进行判断。因为要排序，时间复杂度为O(nlogn)

int hIndex(vector<int>& citations) {
        int n = citations.size();
        vector<int> ct=citations;
	    sort(ct.begin(), ct.end());
	    int ans = 0;
    	for (int i = 0;i < n;i++) {
	    	if (ct[i] <= n - i) {
	    		ans = ct[i];
	    	}
	    	else {
	    		ans=max(ans,n-i);
	    	}
    	}
    	return ans;
    }