Coding interview :逆序對

結合歸併排序的原理計算逆序對數，下面給出代碼

public int inversePair(int[]array){
        if(array==null || array.length <= 1) return 0;

        int[]array2 = new int[array.length];
        return inversePair(array,0,array.length-1,array2);
    }


    public int inversePair(int[] array,int start,int end,int[]array2){
       if(start>=end) return 0;
       int mid = (start+end)/2;

       return inversePair(array,start,mid,array2) + inversePair(array,mid+1,end,array2)
               + merge(array,start,mid+1,end,array2);

    }

    int merge(int[]array,int start,int mid,int end,int[]array2) {

        if(start>=mid){
            return 0;
        }

        int i=start,j=mid,k=start;
        int pairNum = 0;

        while(i<mid && j<=end){
            if(array[i]>array[j]){
                array2[k++] = array[j++];
                pairNum += mid - i;
            }else{
                array2[k++] = array[i++];
            }
        }

        if(i<mid){
            while(i<mid){
                array2[k++] = array[i++];
            }
        }

        if(j<=end){
            while (j<=end){
                array2[k++] = array[j++];
            }
        }

        for(int kk=start;kk<=end;kk++){
            array[kk] = array2[kk];
        }

        return pairNum;
    }