H index II

https://leetcode.com/problems/h-index-ii/

Solution I : Binary search

public class Solution {
    public int hIndex(int[] citations) {
        int start = 0;
        int end = citations.length-1;
        int len = citations.length;
        int result = 0;
        int mid;
        while(start <= end){
            mid = start + (end-start)/2;
            if(citations[mid] >= (len - mid)){
                result = (len-mid); // nice trick if cant find the exact equal condition
                end = mid-1;
            }
            else{
                start = mid + 1;
            }
        }
        return result;
    }
}

Solution II: Standard binary search

Mind the equal condition

public class Solution {
    public int hIndex(int[] citations) {
        if(citations == null || citations.length == 0) return 0;
        int l = 0, r = citations.length;
        int n = citations.length;
        while(l < r){
            int mid = l + (r - l) / 2;
            if(citations[mid] == n - mid) return n - mid;
            if(citations[mid] < citations.length - mid) l = mid + 1;
            else r = mid;
        }
        return n - l;
    }
}

public int hIndex(int[] citations) {
    int l = 0, r = citations.length - 1;
    for (int m = (l + r) / 2; l <= r; m = (l + r) / 2)
        if (citations[m] < citations.length - m) l = m + 1;
        else r = m - 1;
    return citations.length - l;
}

Check out more methods:

https://leetcode.com/discuss/56122/standard-binary-search