归并排序

MergeSort.java

/**原地归并排序
 * Created by dispensable on 16/7/13.
 */
public class MergeSort {

    private static Comparable[] aux;

    public static void merge(Comparable[] a, int lo, int mid, int hi) {
        int i = lo;
        int j = mid+1;

        // 复制到辅助数组
        for (int k = lo; k <= hi; k++) {
            aux[k] = a[k];
        }
        
        //归并
        for (int k = lo; k <= hi; k++) {
            // 左边用完
            if (i > mid) {
                a[k] = aux[j++];
            }
            // 右边用完
            else if (j > hi) {
                a[k] = aux[i++];
            }
            // 左边比右边小
            else if (less(aux[i], aux[j])) {
                a[k] = aux[i++];
            }
            // 左边比右边大
            else {
                a[k] = aux[j++];
            }
        }
    }

    public static void sort(Comparable[] a) {
        aux = new Comparable[a.length];
        topDownSort(a, 0, a.length - 1);
    }

    private static void topDownSort(Comparable[]a, int lo, int hi) {
        if (hi <= lo) return;
        int mid = (lo + hi) / 2;

        topDownSort(a, lo, mid);
        topDownSort(a, mid + 1, hi);
        merge(a, lo, mid, hi);
    }

    private static void downTopSort(Comparable[] a) {

        int N = a.length;
        aux = new Comparable[N];

        for (int size = 1; size < N; size += size) {
            for (int i = 0; i < N - size; i += (size + size)) {
                merge(a, i, i + size - 1, Math.min(i + size + size - 1, N - 1));
            }
        }
    }

    private static boolean less(Comparable a, Comparable b) {
        return a.compareTo(b) < 0;
    }

    private static void show(Comparable[] a) {
        for (int i = 0; i < a.length; i++) {
            System.out.print(a[i]);
        }
        System.out.println();
    }

    public static boolean isSorted(Comparable[] a) {
        for (int i = 1; i < a.length; i++) {
            if (less(a[i], a[i-1])) {
                System.out.println("Not sorted! " + a[i]);
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        System.out.print("\nTopDownMerge: ");
        Integer[] b = {3, 2, 1, 6, 5, 0, 4};
        show(b);
        sort(b);
        isSorted(b);
        show(b);
    }
}

复制到了aux数组就应该用aux[num]来比较，而不是用a