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g0201_0300.s0274_h_index.Solution Maven / Gradle / Ivy

package g0201_0300.s0274_h_index;

// #Medium #Array #Sorting #Counting_Sort #2022_11_05_Time_0_ms_(100.00%)_Space_40.3_MB_(86.98%)

/**
 * 274 - H-Index\.
 *
 * Medium
 *
 * Given an array of integers `citations` where `citations[i]` is the number of citations a researcher received for their ith paper, return compute the researcher's `h` **\-index**.
 *
 * According to the [definition of h-index on Wikipedia](https://en.wikipedia.org/wiki/H-index): A scientist has an index `h` if `h` of their `n` papers have at least `h` citations each, and the other `n − h` papers have no more than `h` citations each.
 *
 * If there are several possible values for `h`, the maximum one is taken as the `h` **\-index**.
 *
 * **Example 1:**
 *
 * **Input:** citations = [3,0,6,1,5]
 *
 * **Output:** 3
 *
 * **Explanation:**
 *
 *     [3,0,6,1,5] 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, their h-index is 3. 
 *
 * **Example 2:**
 *
 * **Input:** citations = [1,3,1]
 *
 * **Output:** 1 
 *
 * **Constraints:**
 *
 * *   `n == citations.length`
 * *   `1 <= n <= 5000`
 * *   `0 <= citations[i] <= 1000`
**/
public class Solution {
    public int hIndex(int[] citations) {
        int len = citations.length;
        int[] freqArray = new int[len + 1];
        for (int citation : citations) {
            freqArray[Math.min(citation, len)]++;
        }
        int totalSoFar = 0;
        for (int k = len; k >= 0; k--) {
            totalSoFar += freqArray[k];
            if (totalSoFar >= k) {
                return k;
            }
        }
        return -1;
    }
}