计算逆序对的算法，算法导论2-4

public class CounttingInversions {
    public static int getInversions(int[] a) {
        return mergeCount(a, 0, a.length - 1);
    }
    private static int mergeCount(int[] a, int from, int to) {
        if (from < to) {
            int middle = (int) Math.floor((from + to) / 2);
            int inL = mergeCount(a, from, middle);
            int inR = mergeCount(a, middle + 1, to);
            int sum = merge(a, from, middle, to);
            return inL + inR + sum;
        }
        return 0;
    }
    private static int merge(int[] array, int from, int middle, int to) {
        int inversionCount = 0;
        int[] leftArray = new int[middle - from + 1];
        int[] rightArray = new int[to - middle];
        for (int i = 0; i < leftArray.length; i++) {
            leftArray[i] = array[from + i];
        }
        for (int i = 0; i < rightArray.length; i++) {
            rightArray[i] = array[middle + i + 1];
        }
        int idxL = 0;
        int idxR = 0;
        for (int idx = from; idx <= to; idx++) {
            if (leftArray[idxL] <= rightArray[idxR]) {
                array[idx] = leftArray[idxL];
                idxL++;
                if (idxL == leftArray.length) {
                    copyRest(array, idx + 1, rightArray, idxR);
                    return inversionCount;
                }
            } else {
                array[idx] = rightArray[idxR];
                inversionCount += leftArray.length - idxL;
                idxR++;
                if (idxR == rightArray.length) {
                    copyRest(array, idx + 1, leftArray, idxL);
                    return inversionCount;
                }
            }
        }
        return inversionCount;
    }
    private static void copyRest(int[] desArr, int dIdx, int[] oriArr, int oIdx) {
        for (; oIdx < oriArr.length; oIdx++) {
            desArr[dIdx] = oriArr[oIdx];
            dIdx++;
        }
    }
    public static void main(String[] args) {
        int[] a = { 4, 3, 2, 1, 0, -1, -2, -3 };
        System.out.println(CounttingInversions.getInversions(a));
    }
}