pat 1129

1129 Recommendation System (25分)

 

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 (≤ 50,000), 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

　

思路：自定义结构体，结构体中value表示item的index,cnt表示item在前面出现的次数。使用set对node自动排序，需要重写set的 < 比较符，实现按照题目要求的输出方式排序，每次输出set中的前k个结构体的value.

注意点：set中元素应该是没有重复的，但是对于结构体，set内部实现去重的时候比较的可能是他们的地址（这个笔者不太了解，欢迎评论区做出品论），因此就算item的value值一样，也不能实现去重，所以每次添加元素的时候，先erase这个value的元素，然后cnt++之后再insert.

代码如下：

#include<cstdio>
#include<set>
#include<cstdlib>
using namespace std;
int book[50001]={0};
typedef struct node{
    int value;
    int cnt;
    bool operator <(const node& a) const{
        return (cnt!=a.cnt)?cnt>a.cnt:value<a.value;
    }
}node;
int main(){
    int n,k;
    scanf("%d %d",&n,&k);
    set<node> s;
    for(int i=0;i<n;i++){
        int value;
        scanf("%d",&value);
        if(i!=0){
            printf("%d:",value);
            int count=0;
            for(set<node>::iterator it=s.begin();count<k&&it!=s.end();it++){
                printf(" %d",(*it).value);
                count++;
            }
            printf("\n");
        }
        set<node>::iterator tempit=s.find(node{value,book[value]});
        if(tempit!=s.end())
        s.erase(tempit);
        ++book[value];
        s.insert(node{value,book[value]});
    }
    
    return 0;
}