Leetcode: H-Index II

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

Hint:

1. Expected runtime complexity is in O(log n) and the input is sorted.

Understand the problem:
Since the problem asks for a O(log n) solution, we can think of using a binary search. 

Code (Java):

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public class Solution {     public int hIndex(int[] citations) {         if (citations == null || citations.length == 0) {             return 0;         }                   int lo = 0;         int hi = citations.length - 1;         int len = citations.length;                   while (lo + 1 < hi) {             int mid = lo + (hi - lo) / 2;                           if (citations[mid] == len - mid) {                 return len - mid;             } else if (citations[mid] < len - mid) {                 lo = mid;             } else {                 hi = mid;             }         }           if (citations[lo] >= len - lo) {             return len - lo;         }                   if (citations[hi] >= len - hi) {             return len - hi;         }                   return 0;     } }