leetcode786 第 K 个最小的素数分数

思路：

二分。

实现：

class Solution
{
public:
    bool check(double x, vector<int>& arr, int k)
    {
        int slow = 0, n = arr.size(), ans = 0;
        for (int i = 1; i < n; i++)
        {
            if (arr[slow] > x * arr[i]) continue;
            int tmp = slow;
            while (slow < i and arr[slow] <= x * arr[i]) slow++;
            ans += (slow - tmp) * (n - i);
        }
        return ans >= k;
    }
    vector<int> kthSmallestPrimeFraction(vector<int>& arr, int k)
    {
        int n = arr.size();
        double l = 0, r = 1, res = -1;
        for (int i = 0; i < 30; i++)
        {
            double m = (l + r) / 2;
            if (check(m, arr, k))
            {
                res = m; r = m;
            }
            else l = m;
        }
        double minn = 0x3f3f3f3f;
        int a = -1, b = -1;
        for (int i = 0; i < n; i++)
        {
            double x = res * arr[i];
            int p = upper_bound(arr.begin(), arr.begin() + i, x) - arr.begin();
            if (p)
            {
                p--;
                double tmp = (double)arr[p] / arr[i];
                if (abs(res - tmp) < minn)
                {
                    minn = abs(res - tmp);
                    a = p; b = i;
                }
            }
        }
        return vector<int>{arr[a], arr[b]};
    }
};