H-Index II

描述

Follow up for H-Index: What if the citations array is sorted in ascending order? Could you optimize your algorithm?

分析

设数组长度为n，那么n-i就是引用次数大于等于nums[i]的文章数。如果nums[i]<n-i，说明i是有效的H-Index, 如果一个数是H-Index，那么最大的H-Index一定在它后面（因为是升序的），根据这点就可以进行二分搜索了。

代码

// H-Index II
// Time complexity: O(logn), Space complexity: O(1)
public class Solution {
    public int hIndex(int[] citations) {
        final int n = citations.length;
        int begin = 0;
        int end = citations.length;

        while (begin < end) {
            final int mid = begin + (end - begin) / 2;
            if (citations[mid] < n - mid) {
                begin = mid + 1;
            } else {
                end = mid;
            }
        }
        return n - begin;
    }
}