java 归并排序

Posted 2020-12-14 懒惰的星期六

　　把数据分为两段，从两段中逐个选最小的元素移入新数据段的末尾。

　　可从上到下或从下到上进行。

过程演示：

一、迭代法
1.待排序数据为data
2.每轮排序的结果保存为result
3.每轮排序的结果result需要重新复制给data
public static void main(String[] args) {

        // TODO Auto-generated method stub
        ArrayList<Integer> data = new ArrayList<>();
        Random random = new Random(42);
        for (int i = 0; i < 10; i++) {
            data.add((int)(random.nextInt(100)));
        }
        Iterator<Integer> iterator = data.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }
        System.out.println();
        ArrayList<Integer> result = new ArrayList<>();
        result.addAll(data);
        for (int seg = 1; seg < data.size(); seg=2*seg) {
            for (int start = 0; start < data.size(); start=start+2*seg) {
                int start1,end1,start2,end2;
                int mid = Math.min(start+seg,data.size());
                int end = Math.min(start+2*seg,data.size());
                start1 = start; end1=mid; start2=mid; end2 = end;
                int k=start,idx;
                while(start1<end1 && start2<end2){
//                    idx = data.get(start1)<data.get(start2)?start1++:start2++;
//                    result.set(k++,data.get(idx));
                    if (data.get(start1)<data.get(start2))
                        result.set(k++,data.get(start1++));
                    else
                        result.set(k++,data.get(start2++));
                }
                while(start1<end1){
                    result.set(k++,data.get(start1++));
                }
                while(start2<end2){
                    result.set(k++,data.get(start2++));
                }
            }
            data.clear();
            data.addAll(result);
        }
        for (Iterator<Integer> iterator2 = result.iterator(); iterator2.hasNext();) {
            Integer integer = (Integer) iterator2.next();
            System.out.println(integer);
            
        }
    }