hihoCoder#1133 二分·二分查找之k小数

原题地址

经典问题了，O(n)时间内找第k大的数

代码：

 #include <iostream>

 using namespace std;

 int N, K;
 int *a;

 int search(int left, int right, int k) {
   if (left > right)
     return -;
   int l = left;
   int r = right;
   int p = a[left];
   while (l < r) {
     while (l < r && a[r] > p)
       r--;
     a[l] = a[r];
     while (l < r && a[l] < p)
       l++;
     a[r] = a[l];
   }
   a[l] = p;
   if ((l - left + ) == k)
     return a[l];
   else if ((l - left + ) < k)
     return search(l + , right, k - (l - left + ));
   else
     return search(left, l - , k);
 }

 int main() {
   scanf("%d%d", &N, &K);
   a = new int[N];
   for (int i = ; i < N; i++)
     scanf("%d", &(a[i]));
   printf("%d\n", search(, N - , K));
   delete a;
   return ;
 }