无序数组求第K大/第K小的数

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<string>
 4 #include<cstring>
 5 #include<cmath>
 6 #include<algorithm>
 7 using namespace std;
 8 int n, k;
 9 int a[100010];
10 int par(int l, int r)
11 {
12     int t = a[l];
13     while (l < r)
14     {
15         while (l < r &&a[r] <= t)    r--;
16         a[l] = a[r];
17         while (l < r &&a[l] >= t)    l++;
18         a[r] = a[l];
19     }
20     a[l] = t;
21     return l;
22 }
23 int search(int l, int r,int k)
24 {
25     if (l <= r)
26     {
27         int p = par(l, r);
28         if (p - l + 1 == k)    return p;
29         else if (p - l + 1 < k)    return search(p+1,r,k-(p-l+1));
30         else return search(l,p-1,k);
31     }
32 }
33 int main()
34 {
35     cin >> n >> k;
36     for (int i = 1; i <= n; i++)
37         cin >> a[i];
38     cout << a[search(1, n, k)] << endl;
39 
40     return 0;
41 }