【两次过】【Comparator】Lintcode 613. 优秀成绩

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本文介绍了一种算法，用于处理学生考试成绩数据，通过两种方法找出每位学生最高五个分数的平均值，一是通过排序，二是利用优先队列实现更高效的数据处理。

每个学生有两个属性 id 和 scores。找到每个学生最高的5个分数的平均值。

样例

例1:

输入: 
[[1,91],[1,92],[2,93],[2,99],[2,98],[2,97],[1,60],[1,58],[2,100],[1,61]]
输出:
1: 72.40
2: 97.40

例2:

输入:
[[1,90],[1,90],[1,90],[1,90],[1,90],[1,90]]
输出: 
1: 90.00

解题思路1：

排序。按照id升序再按照score降序，最后再依次取每个id的前5项。

/**
 * Definition for a Record
 * class Record {
 *     public int id, score;
 *     public Record(int id, int score){
 *         this.id = id;
 *         this.score = score;
 *     }
 * }
 */
public class Solution {
    /**
     * @param results a list of <student_id, score>
     * @return find the average of 5 highest scores for each person
     * Map<Integer, Double> (student_id, average_score)
     */
    public Map<Integer, Double> highFive(Record[] results) {
        // Write your code here
        Map<Integer, Double> map = new HashMap<>();
        if(results == null || results.length < 5)
            return map;
        
        Arrays.sort(results, new Comparator<Record>(){
            @Override
            public int compare(Record a, Record b){
                if(a.id != b.id)
                    return a.id - b.id;
                else
                    return b.score - a.score;
            }
        });
        
        int reId = results[0].id;
        for(int i=0; i<results.length;){
            int times = 0;
            int sum = 0;
            
            while(i < results.length && results[i].id == reId && times < 5){
                sum += results[i].score;
                times++;
                i++;
            }
            
            if(sum != 0){
                map.put(reId, (double)sum/5);
                reId++;
            }else
                i++;
        }
        
        return map;
    }
}

解题思路2：

采用优先队列。使用Map<Integer, PriorityQueue<Integer>>来存储id以及id对应的score集合，始终保持优先队列只存储前5大的元素即可。

/**
 * Definition for a Record
 * class Record {
 *     public int id, score;
 *     public Record(int id, int score){
 *         this.id = id;
 *         this.score = score;
 *     }
 * }
 */
public class Solution {
    /**
     * @param results a list of <student_id, score>
     * @return find the average of 5 highest scores for each person
     * Map<Integer, Double> (student_id, average_score)
     */
    public Map<Integer, Double> highFive(Record[] results) {
        // Write your code here
        Map<Integer, Double> res = new HashMap<>();
        if(results == null || results.length < 5)
            return res;
        
        Map<Integer, PriorityQueue<Integer>> map = new HashMap<>();
        for(int i=0; i<results.length; i++){
            if(!map.containsKey(results[i].id))
                map.put(results[i].id, new PriorityQueue<Integer>());
            
            PriorityQueue<Integer> queue = map.get(results[i].id);
            if(queue.size() < 5)
                queue.add(results[i].score);
            else{
                if(results[i].score > queue.peek()){
                    queue.poll();
                    queue.add(results[i].score);
                }
            }
        }
        
        for(Integer id : map.keySet()){
            PriorityQueue<Integer> queue = map.get(id);
            double sum = 0;
            while(!queue.isEmpty())
                sum += queue.poll();
            
            res.put(id, sum/5.0);
        }
        
        return res;
    }
}