四 3745. 牛的学术圈 I (双指针|二分)

3745. 牛的学术圈 I (双指针|二分)

思路：要到达最大h指数，此时有a1篇引用大于等于h，a2篇引用等于h-1，且满足a1+min(a2, L)大于等于h，a1、a2可使用双指针分别获取，h则可使用二分法遍历减少耗时。

import java.util.*;

public class Main {
    private static int N, L;
    private static int[] a;

    private static boolean check(int mid) {
        int a1 = 0, a2 = 0;
        for (int i = 1; i <= N; i++) {
            if (a[i] >= mid) {
                a1++;
            }
            else if (a[i] == mid - 1) {
                a2++;
            }
        }
        return a1 + Math.min(a2, L) >= mid;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        N = sc.nextInt();
        L = sc.nextInt();
        a = new int[N + 2];
        for (int i = 1; i <= N; i++) {
            a[i] = sc.nextInt();
        }

        int left = 0, right = N;
        while (left < right) {
            int mid = left + right + 1 >> 1;
            if (check(mid)) {
                left = mid;
            }
            else {
                right = mid - 1;
            }
        }
        System.out.println(right);
    }
}