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.

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:

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

Solution

O(nlogn)时间，O(1)空间。现将数组排序。然后从大到小遍历，一边计数一边比较计数与当前的引用数，直到计数大于引用数。

``````public class Solution {
public int hIndex(int[] citations) {
int len = citations.length;
Arrays.sort(citations);
int result = 0;
for (int i = citations.length - 1; i >= 0; i--) {
if (result >= citations[i]) {
return result;
}
result++;
}

return result;
}
}
``````

O(n)时间，O(n)空间。

``````public class Solution {
public int hIndex(int[] citations) {
int len = citations.length;
if(citations == null || len == 0) return 0;
int[] counts = new int[len + 1];
for(int c : citations){
if(c > len) counts[len]++;
else counts[c]++;
}
if(counts[len] >= len) return len;
for(int i = len - 1; i >= 0; i--){
counts[i] += counts[i + 1];
if(counts[i] >= i) return i;
}
return 0;
}
}
``````