牛客网真题-45-孙悟空的徒弟

没人写题解嘛，暴力法，枚举全部情况，放入容器中 65%
然后二分法(借鉴抄袭大佬的)，不知道具体的值，也能用二分，具体的看代码吧。

import java.util.*;
public class Main {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt(), k = scanner.nextInt();
        Integer[] ints = new Integer[n];
        for(int i = 0; i < n; i++){
            ints[i] = scanner.nextInt();
        }

//            //暴力法 65%
//            ArrayList<Integer> arrayList = new ArrayList<>();
//            for(int i = 0; i < ints.length; i++){
//                for(int j = i + 1; j < ints.length; j++){
//                    arrayList.add(ints[i] * ints[j]);
//                }
//            }
//            arrayList.sort((i, j) -> Integer.compare(j, i));
//            System.out.println(arrayList.get(k - 1));
//            //
//        }
        Arrays.sort(ints);
        long minP = 0, maxP = ints[n - 1] * ints[n - 2];
        while (minP < maxP) {
            long mid = (minP + maxP + 1) >> 1;
            long cnt = 0;
            int low = 0, high = n - 1;
            //找出比mid大的数
            while (low < high && cnt < k) {
                while (low < high && ints[low] * ints[high] < mid) {
                    ++low;
                }
                cnt += Math.max(high - low, 0);
                --high;
            }
            //剪枝
            if(cnt >= k){
                minP = mid;
            }else{
                maxP = mid - 1;
            }
        }
        System.out.println(minP);
    }
}