1129 recommendation system set

Recommendation system predicts the preference that a user would give to an item. Now you are asked to program a very simple recommendation system that rates the user’s preference by the number of times that an item has been accessed by this user.

Input Specification:

Each input file contains one test case. For each test case, the first line contains two positive integers: N (<= 50000), the total number of queries, and K (<= 10), the maximum number of recommendations the system must show to the user. Then given in the second line are the indices of items that the user is accessing – for the sake of simplicity, all the items are indexed from 1 to N. All the numbers in a line are separated by a space.

Output Specification:

For each case, process the queries one by one. Output the recommendations for each query in a line in the format:

query: rec[1] rec[2] … rec[K]

where query is the item that the user is accessing, and rec[i] (i = 1, … K) is the i-th item that the system recommends to the user. The first K items that have been accessed most frequently are supposed to be recommended in non-increasing order of their frequencies. If there is a tie, the items will be ordered by their indices in increasing order.

Note: there is no output for the first item since it is impossible to give any recommendation at the time. It is guaranteed to have the output for at least one query.

Sample Input:
12 3
3 5 7 5 5 3 2 1 8 3 8 12
Sample Output:
5: 3
7: 3 5
5: 3 5 7
5: 5 3 7
3: 5 3 7
2: 5 3 7
1: 5 3 2
8: 5 3 1
3: 5 3 1
8: 3 5 1
12: 3 5 8

#include <iostream>
#include <set>
#include <algorithm>
using namespace std;
int book[50001];//存储cnt
//思路：采用在线处理 每输入一个数字 遍更新其频率并再排序输出(set会根据大小自动排序)
struct node{
    int value;
    int cnt;
    bool operator < (const node &a) const {//运算符重载
        return (cnt != a.cnt) ? cnt > a.cnt : value < a.value;
    }
};

int main(){
    int n, k, t;
    cin >> n >> k;
    set<node> s;
    for (int i = 0; i < n; i++) {
        cin >> t;
        if (i) {
            printf("%d:", t);
            int c = 0;
            for (auto it = s.begin(); c < k && it != s.end(); it++) {
                printf(" %d", it->value);
                c++;
            }
            printf("\n");
        }
        //删除这个元素 将cnt+1后 再把这个元素插入set  set在插入时会自动排序
        auto it = s.find(node{t, book[t]});
        if (it != s.end()) s.erase(it);
        book[t]++;
        s.insert(node{t, book[t]});
    }
    return 0;
}