选择出数列中前k个最大的数。
这里因为数据特殊，所以可以使用hash表的方法：
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#include
#include
#include
#include
using namespace std; const int SIZE = 1000005; const int SMALL = -500000; bool arr[SIZE]; int main() { int n, m, a, maxN = INT_MIN; while (~scanf("%d %d", &n, &m)) { for (int i = 0; i < n; i++) { scanf("%d", &a); arr[a - SMALL] = true; if (a - SMALL > maxN) maxN = a - SMALL; } for (int i = maxN; i >= 0 && m; i--) { if (arr[i]) { if (m > 1) printf("%d ", i + SMALL); else printf("%d\n", i + SMALL); m--; } } } return 0; }
也可以使用选择前k个数，然后排序，不过难度高很多：
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#include
#include
#include
#include
using namespace std; int partition(int *arr, int low, int up) { for (int i = low; i < up; i++) if (arr[i] >= arr[up]) swap(arr[low++], arr[i]); swap(arr[low], arr[up]); return low; } //K作为绝对下标位置处理 void randSelectK(int *arr, int low, int up, int K) { int r = low + rand() % (up - low + 1); swap(arr[r], arr[up]); int idx = partition(arr, low, up); if (idx > K) randSelectK(arr, low, idx-1, K); else if (idx < K) randSelectK(arr, idx+1, up, K); } void randQuickSort(int *arr, int low, int up) { if (low >= up) return ;//不能是low == up int r = low + rand() % (up - low + 1); swap(arr[r], arr[up]); int mid = partition(arr, low, up); randQuickSort(arr, low, mid-1); randQuickSort(arr, mid+1, up); } const int SIZE = 1000000; int arr[SIZE]; int main() { int n, m; srand(unsigned(time(NULL))); while(~scanf("%d %d", &n, &m)) { for (int i = 0; i < n; i++) scanf("%d", &arr[i]); randSelectK(arr, 0, n-1, m-1); randQuickSort(arr, 0, m-1); for (int i = 0; i < m-1; i++) { printf("%d ", arr[i]); } printf("%d\n", arr[m-1]); } return 0; }
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