274. H-Index

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."

Example:

Input: citations = [3,0,6,1,5]
Output: 3 
Explanation: [3,0,6,1,5] 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, her h-index is 3.

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

public class Solution {
    public int hIndex(int[] citations) {
        ArrayList<Integer> list = new ArrayList();
        for(int i: citations){
            list.add(i);
        }
        Collections.sort(list);
        Collections.reverse(list);
        for (int i = 0; i < citations.length; ++i) {
            if (i + 1 == list.get(i)) return i+1;
            if (i + 1 > list.get(i)) return i; //[0]的情况，很傻逼
        }
        return citations.length;
    }
}

这个方法反而慢的一比

public class Solution {
    public int hIndex(int[] citations) {
        Arrays.sort(citations);
        reverse(citations);
        for (int i = 0; i < citations.length; ++i) {
            if (i + 1 == citations[i]) return i+1;
            if (i + 1 > citations[i]) return i; //[0]的情况，傻逼
        }
        return citations.length;
    }
    private static void reverse(int[] nums) {
        int left = 0;
        int right = nums.length - 1;
        while (left < right) {
            final int tmp = nums[left];
            nums[left] = nums[right];
            nums[right] = tmp;
            ++left;
            --right;
        }
    }
}

先从大到小排序，然后从前往后扫描，如果当前文章数（即当前下标+1）等于值本身，则返回当前文章数作为 h-index；

如果当前文章数大于值本身，则返回当前文章数-1作为H-Index, 因为当前文章的引用数小于当前文章数，不能算在内。时间复杂度O(nlog，空间复杂度O(1)。