hihocoder1133 二分·二分查找之k小数

Posted 2020-08-30 王宜鸣

思路：

类似于快排的分治算法。

实现：

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <algorithm>
 4 #include <cstring>
 5 using namespace std;
 6 
 7 const int MAXN = 1000000;
 8 int a[MAXN + 5], n, k;
 9 
10 int partition(int * num, int l, int r)
11 {
12     int i = l, j = r, m = (l + r) >> 1;
13     int x = num[m];
14     while (i != j)
15     {
16         while (j > m && num[j] >= x)
17         {
18             j--;
19         }
20         num[m] = num[j], m = j;
21         while (i < m && num[i] <= x)
22         {
23             i++;
24         }
25         num[m] = num[i], m = i;
26     }
27     num[i] = x;
28     return i;
29 }
30 
31 int findK(int * num, int l, int r, int k)
32 {
33     int pos = partition(num, l, r);
34     if (pos == k)
35     {
36         return num[k];
37     }
38     if (pos > k)
39     {
40         return findK(num, l, pos - 1, k);
41     }
42     return findK(num, pos + 1, r, k);
43 }
44 
45 int main()
46 {
47     scanf("%d %d", &n, &k);
48     for (int i = 1; i <= n; i++)
49     {
50         scanf("%d", &a[i]);
51     }
52     printf("%d\n", findK(a, 1, n, k));
53     return 0;
54 }