1129. Recommendation System (25)
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 12Sample 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
提交代码
刚开始想的是每一轮都用sort排下序，结果超时，之后看了别人的题解发现要用set，而且我竟然才知道set可以通过结构体优先级设置来自己定义排序规则，之前用优先队列的时候用过重载运算符，但是都忘了，算是今天又复习了一遍。附上代码
#include<bits/stdc++.h>
using namespace std;
struct node{int v;int cnt;friend bool operator < (const node &x,const node &y){if(x.cnt!=y.cnt) return x.cnt>y.cnt;else return x.v<y.v;}
};
set<node>s;
int n,k,a[50005],visit[50005];
int main(){scanf("%d%d",&n,&k);for(int i=1;i<=n;i++)scanf("%d",&a[i]);node temp;for(int i=2;i<=n;i++){int x=a[i-1];if(!visit[x]){visit[x]=1;temp.v=x;temp.cnt=1;s.insert(temp);}else{for(auto it=s.begin();it!=s.end();it++){node now=*it;if(now.v==x){temp.v=x;temp.cnt=now.cnt+1;s.erase(it);s.insert(temp);break;}}}printf("%d: ",a[i]);int l=s.size();int len=min(l,k);int j=1;for(auto it=s.begin();it!=s.end()&&j<=len;it++){node now=*it;printf("%d",now.v);if(j!=len)printf(" ");j++;}printf("\n");}return 0;
}