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

  1. // H-Index II
  2. // Time complexity: O(logn), Space complexity: O(1)
  3. public class Solution {
  4.     public int hIndex(int[] citations) {
  5.         final int n = citations.length;
  6.         int begin = 0;
  7.         int end = citations.length;
  9.         while (begin < end) {
  10.             final int mid = begin + (end - begin) / 2;
  11.             if (citations[mid] < n - mid) {
  12.                 begin = mid + 1;
  13.             } else {
  14.                 end = mid;
  15.             }
  16.         }
  17.         return n - begin;
  18.     }
  19. }

原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/java/search/h-index-ii.html