H-Index.js

/**
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:

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

*/
/**
 * @param {number[]} citations
 * @return {number}
 */
var hIndex = function(citations) {
    var len = citations.length,
        i;
    
    citations.sort(function (a, b) {
        if (a < b) {
            return 1;
        }
        return -1;
    });
    
    for (i = 0; i < len; i++) {
        if (citations[i] <= i) {
            return i;
        }
    }
    
    return len;
};


/**
SOLUTION 2:
复杂度
时间 O(N) 空间 O(N)

思路
也可以不对数组排序，我们额外使用一个大小为N+1的数组stats。stats[i]表示有多少文章被引用了i次，这里如果一篇文章引用大于N次，我们就将其当为N次，因为H指数不会超过文章的总数。为了构建这个数组，我们需要先将整个文献引用数组遍历一遍，对相应的格子加一。统计完后，我们从N向1开始遍历这个统计数组。如果遍历到某一个引用次数时，大于或等于该引用次数的文章数量，大于引用次数本身时，我们可以认为这是H指数。之所以不用再向下找，因为我们要取最大的H指数。那如何求大于或等于某个引用次数的文章数量呢？我们可以用一个变量，从高引用次的文章数累加下来。因为我们知道，如果有x篇文章的引用大于等于3次，那引用大于等于2次的文章数量一定是x加上引用次数等于2次的文章数量。

代码
public class Solution {
    public int hIndex(int[] citations) {
        int[] stats = new int[citations.length + 1];
        int n = citations.length;
        // 统计各个引用次数对应多少篇文章
        for(int i = 0; i < n; i++){
            stats[citations[i] <= n ? citations[i] : n] += 1;
        }
        int sum = 0;
        // 找出最大的H指数
        for(int i = n; i > 0; i--){
            // 引用大于等于i次的文章数量，等于引用大于等于i+1次的文章数量，加上引用等于i次的文章数量 
            sum += stats[i];
            // 如果引用大于等于i次的文章数量，大于引用次数i，说明是H指数
            if(sum >= i){
                return i;
            }
        }
        return 0;
    }
}

*/