1亿条数据中获取最大100条，论坛找来的代码TreeSet 和另外一种方法-优快云博客

本文链接：https://blog.youkuaiyun.com/iteye_5474/article/details/81969631

本文介绍了一种从大量整数中筛选出最大的100个数的算法实现，并通过两个不同方法进行了对比。使用TreeSet进行动态维护，确保集合大小始终保持在100个元素，同时提供了一个备选方案利用ArrayList实现类似功能。

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import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Random; 
import java.util.Set; 
import java.util.TreeSet; 


class TreeSF {

	public static Set<Integer> getTop100(int[] inputArray) {

		TreeSet<Integer> top100 = new TreeSet();
		for (int i = 0; i < inputArray.length; i++) {

			if (top100.size() < 100) {
				top100.add(inputArray[i]);
			} else if ((Integer) top100.first() < inputArray[i]) {
				Object obj = top100.first();
				top100.remove(obj);
				top100.add(inputArray[i]);
			}

		}
		//System.out.println(top100.toString());
		return top100;

	}

	public static void main(String[] args) {

		int numberCount = 100000000;//0;

		int maxNumber = numberCount;

		int inputArray[] = new int[numberCount];

		Random random = new Random();

		for (int i = 0; i < numberCount; ++i) {

			inputArray[i] = Math.abs(random.nextInt(maxNumber));

		}

		System.out.println("Sort begin...");

		long current = System.currentTimeMillis();

		Set<Integer> result = TreeSF.getTop100(inputArray);

		System.out.println("Spend time:"
				+ (System.currentTimeMillis() - current));

	}

}
class findValue {

	public static void main(String[] args){

		int max = 10000*1000;
		int length=100;
		int ints[] = new int[max];
		Random random = new Random();
		for (int i=0;i<max;i++) {
			ints[i]=Math.abs(random.nextInt(max));
		}
		List<Integer> list = new ArrayList();

		long start = System.currentTimeMillis();
		int size=0;
		int value=0;
		list.add(ints[0]);
		for(int i=1;i<max;i++){
			value=ints[i];
			size=list.size();
			if(value<list.get(size-1)){
				if(size<length){
					list.add(value);
				}
				continue;
			}else if(value>list.get(0)){
				list.add(0,value);
				if(size==length){
					list.remove(size);
				}
				continue;
			}

			for(int j=0;j<size;j++){
				if(value>list.get(j)){
                                              //如果不需要排除重复数字，则去掉该判断
					if(j>0&&value!=list.get(j-1)){
						list.add(j,value);
						if(size==length){
						  list.remove(size);
						}
					}
					break;
				}
			 }
		}

		System.out.println("time:"+(System.currentTimeMillis()-start));

		System.out.println(list);
		TreeSet<Integer> set = new TreeSet();
		set.addAll(list);

		System.out.println(set.size()+":"+list.size());
		if(set.size()!=list.size()){
			System.out.println("error");
		}
		int i=0;
		for(Iterator<Integer> it=set.iterator();it.hasNext();){
			value=it.next();
			System.out.println(value+":"+list.get(list.size()-1-i));
			if(value!=list.get(list.size()-1-i)){
				System.out.println("error");
				break;
			}
			i++;
		}
	}
}