POJ 2299 Java

这题的题目挺难理解的，画的图让我想到冒泡排序，但是用冒泡肯定会超时的，后来经过高人指点才知道要用归并排序，但是知道了方法还是不会求逆序数，没办法，参考了网上大牛的解放之后自己写的代码如下:

import java.util.Scanner;public class Main {//记录逆序数的个数private static long sum=0;public static void mergeSort(long[] a,long[] temp,int left,int right){if(left==right){return;}if((right-left)==1){if(a[left]>a[right]){long b=a[left];a[left]=a[right];a[right]=b;sum++;}return;}else{int mid=(left+right)/2;mergeSort(a,temp,left,mid);mergeSort(a,temp,mid+1,right);for(int i=left;i<=right;i++){temp[i]=a[i];}int i1=left;int i2=mid+1;for(int curr=left;curr<=right;curr++){if(i1==mid+1){a[curr]=temp[i2++];}else{if(i2>right){a[curr]=temp[i1++];}else{if(temp[i1]<temp[i2]){a[curr]=temp[i1++];}else{//mid右边的都是逆序的sum+=mid-i1+1;a[curr]=temp[i2++];}}}}}}/** * @param args */public static void main(String[] args) {// TODO Auto-generated method stublong[] a=new long[500000];long[] temp=new long[500000];Scanner reader=new Scanner(System.in);while(reader.hasNext()){long numOfLine=reader.nextLong();if(numOfLine!=0){for(int i=0;i<numOfLine;i++){a[i]=reader.nextLong();//System.out.print(a[i]+" ");}//System.out.println();mergeSort(a,temp,0,(int) (numOfLine-1));//System.out.print("sorted:");//for(int i=0;i<numOfLine;i++){////System.out.print(a[i]+" ");////}//System.out.println();System.out.println(sum);sum=0;}else{break;}}}}