(1129) Recommendation System 排序

谁借莪１个温暖的怀抱￠ 2022-03-18 06:51 190阅读 0赞

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

解题思路:

利用数组存储当前读入item前各item的出现次数,把这些元素输入vector中(无重复),调用sort函数排序

这题的坑点是元素数量大的话排序会超时,所以我们要把vector内元素的容量控制在10以内

#define _CRT_SECURE_NO_WARNINGS
    #include <iostream>
    #include <algorithm>
    #include <vector>
    #include <set>
    using namespace std;
    const int MAXN = 50010;
    int N, K;
    vector<int> curAppearElements;
    int appearTimes[MAXN];
    
    bool cmp(int a, int b) {
    	if (appearTimes[a] != appearTimes[b]) {
    		return appearTimes[a] > appearTimes[b];
    	}
    	else {
    		return a < b;
    	}
    }
    
    int main() {
    	fill(appearTimes, appearTimes + MAXN, 0);
    	scanf("%d %d", &N, &K);
    	int element;
    	for (int i = 1; i <= N; ++i) {
    		scanf("%d", &element);
    		if (i == 1) {
    			curAppearElements.push_back(element);
    			appearTimes[element]++;
    			continue;
    		}
    		
    		printf("%d:", element);
    		for (int j = 0; j < curAppearElements.size(); ++j) {
    			if (j >= K) break;
    			printf(" %d", curAppearElements[j]);
    		}
    		if (appearTimes[element] == 0) {
    			curAppearElements.push_back(element);
    		}
    		appearTimes[element]++;
    		sort(curAppearElements.begin(), curAppearElements.end(), cmp);
    		if (curAppearElements.size() > 10) curAppearElements.pop_back();
    		printf("\n");
    	}
    	system("PAUSE");
    	return 0;
    }