结合归并排序的原理计算逆序对数,下面给出代码
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;
}