hihoCoder#1133 二分·二分查找之k小数

 1 #include <iostream>
 2 
 3 using namespace std;
 4 
 5 int N, K;
 6 int *a;
 7 
 8 int search(int left, int right, int k) {
 9   if (left > right)
10     return -1;
11   int l = left;
12   int r = right;
13   int p = a[left];
14   while (l < r) {
15     while (l < r && a[r] > p)
16       r--;
17     a[l] = a[r];
18     while (l < r && a[l] < p)
19       l++;
20     a[r] = a[l];
21   }
22   a[l] = p;
23   if ((l - left + 1) == k)
24     return a[l];
25   else if ((l - left + 1) < k)
26     return search(l + 1, right, k - (l - left + 1));
27   else
28     return search(left, l - 1, k);
29 }
30 
31 int main() {
32   scanf("%d%d", &N, &K);
33   a = new int[N];
34   for (int i = 0; i < N; i++)
35     scanf("%d", &(a[i]));
36   printf("%d\n", search(0, N - 1, K));
37   delete a;
38   return 0;
39 }