题目：
Given an array of citations **sorted in ascending order **(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."
Input: citations = [0,1,3,5,6]
Output: 3
Explanation: [0,1,3,5,6] means the researcher has 5 papers in total and each of them had received 0, 1, 3, 5, 6 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.
Follow up:
- This is a follow up problem to H-Index, where
citationsis now guaranteed to be sorted in ascending order. - Could you solve it in logarithmic time complexity?
思路：
此题采用二分查找解决。
- 首先定位中间的位置。如果中间位置元素的值大于它之后元素的数。则在左半区中查找答案。
- 如果小于之后的元素数则在右半区查找答案。因为最后需要找到最大的元素。
- 在左半区查找的同时还需要记录大于指定元素的元素个数，记为result，即为最终的结果。
代码：
public int hIndex(int[] citations) {
if(citations == null || citations.length == 0)
return 0;
int result = 0;
int length = citations.length;
int left = 0;
int right = citations.length - 1;
while(left <= right){
int mid = left + (right - left)/2;
if(citations[mid] >= length - mid){
result = length - mid;
right = mid - 1;
}else{
left = mid + 1;
}
}
return result;
}
