Formatted question description:

 275	H-Index II

 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.


 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 citations is now guaranteed to be sorted in ascending order.
 Could you solve it in logarithmic time complexity?


First initialize left and right to 0 and len-1, then take the middle value mid, compare citations[mid] with len-mid,

  • If the former is big, right moves before mid
  • On the contrary, right moves after mid The loop condition is left<=right, and finally returns len-left.



public class H_Index_II {
    class Solution {
        public int hIndex(int[] citations) {
            int len = citations.length, left = 0, right = len - 1;
            while (left <= right) {
                int mid = (left + right) / 2;
                if (citations[mid] == len - mid) {
                    return len - mid;
                else if (citations[mid] > len - mid) { // cites > h`, try next round
                    right = mid - 1;
                else {
                    left = mid + 1;

            return len - left;

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