Java范例从20亿个随机整数中找出重复次数最多的1000个整数只需要8秒的时间

最新推荐文章于 2025-11-07 00:15:00 发布

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#找重复次数最多的整数

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本范例代码运行20亿个整数找重复次数最多的1000个整数大约需要5GB的内存

package com.contoso;

import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Random;

// 本范例代码运行20亿个整数找重复次数最多的1000个整数大约需要5GB的内存
public class TopIntTimes {

    public static void main(String[] args) {
        int[] data = new int[200000000];
        for (int i = 0; i < data.length; i++) {
            data[i] = new Random().nextInt(Integer.MAX_VALUE);
        }

        int[] hash = new int[Integer.MAX_VALUE / 16 + Integer.MAX_VALUE % 16];
        Map<Integer, Integer> countMap = new HashMap();
        long startTime = System.nanoTime();

        for (int i = 0; i < hash.length; i++) {
            hash[i] = 0;
        }

        for (int number : data) {

            int times = (hash[number >>> 4] >>> (2 * (number % 16))) & (1 | 1 << 1);
            //if(times > 0 ){
            //    System.out.println(times);
            //}

            if (times <= ((1 | 1 << 1) - 1)) {
                times++;

                hash[number >>> 4] &= ~((1 | 1 << 1) << (2 * (number % 16)));

                hash[number >>> 4] |= times << (2 * (number % 16));

                if ((1 | 1 << 1) == times) {
                    countMap.put(number, times);
                }
            } else {

                if ((1 | 1 << 1) == times) {
                    countMap.put(number, countMap.get(number).intValue() + 1);
                }
            }
        }

        List<Counter> counterList = new LinkedList<Counter>();
        for (Integer number : countMap.keySet()) {
            counterList.add(new Counter(countMap.get(number), number));
        }
        Collections.sort(counterList);
        Integer ROWS = 1000;
        for (Counter counter : counterList) {
            ROWS--;
            System.out.println(counter.getNumber() + "----" + counter.getCounter());
            if (ROWS == 0) {
                break;
            }
        }
        long endTime = System.nanoTime();
        double timeTaken = (endTime - startTime) / 1e9;
        System.out.println("Time Taken in seconds：" + timeTaken);
    }
}

class Counter implements Comparable<Counter> {

    private Integer counter;
    private Integer number;

    public Counter(Integer counter, Integer number) {
        this.counter = counter;
        this.number = number;
    }

    public Integer getCounter() {
        return this.counter;
    }

    public Integer getNumber() {
        return this.number;
    }

    @Override
    public int compareTo(Counter counter) {
        return counter.getCounter().compareTo(this.getCounter());
    }
}

run:
1955171379----6
1619319205----6
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Time Taken in seconds：7.784759607
BUILD SUCCESSFUL (total time: 22 seconds)