Binary Search | Jes2ica.IO

Problem

Given an array of citations (each citation is a non-negative integer) of a researcher, write a function to compute the researcher’s h-index (citations array is sorted in ascending order).

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.”

For example, given citations = [3, 0, 6, 1, 5], which means the researcher has 5 papers in total and each of them had received 3, 0, 6, 1, 5 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, his h-index is 3.

Note: If there are several possible values for h, the maximum one is taken as the h-index.

Solution

public int hIndex(int[] citations) {
    int left = 0, right = citations.length;
    // For case "[0]", left = right.
    while (left <= right) {
        int mid = left + (right - left) / 2;
        if (citations.length - mid > citations[mid]) {
            // Move left pointer.
            left = mid + 1;
        } else {
            right = mid;
        }
    }
    return citations.length - left;
}

Notes

Arrays.binarySearch() returns a negative value that is equivalent to -[insertion point] - 1, where insertion point is defined as the index at which the search key would be inserted into the array or list.