H-Index II
題意：
Follow up for H-Index: What if the citations array is sorted in ascending order? Could you optimize your algorithm?
Hint:
Expected runtime complexity is in O(log n) and the input is sorted.
解題思路：
使用二分搜尋法，程式碼如下：
public class Solution {
public int hIndex(int[] citations) {
if (citations == null || citations.length == 0) {
return 0;
}
int len = citations.length;
int start = 0;
int end = citations.length - 1;
while (start <= end) {
int mid = start + (end - start) / 2;
if (citations[mid] >= len - mid) {
end = mid - 1;
} else {
start = mid + 1;
}
}
return len - start;
}
}