POJ——2299（Ultra-QuickSort）树状数组求逆序数

In this problem, you have to analyze a particular sorting algorithm. The algorithm processes a sequence of n distinct integers by swapping two adjacent sequence elements until the sequence is sorted in ascending order. For the input sequence 

9 1 0 5 4                                                        ,

Ultra-QuickSort produces the output 

0 1 4 5 9 .

Your task is to determine how many swap operations Ultra-QuickSort needs to perform in order to sort a given input sequence.

Input

The input contains several test cases. Every test case begins with a line that contains a single integer n < 500,000 -- the length of the input sequence. Each of the the following n lines contains a single integer 0 ≤ a[i] ≤ 999,999,999, the i-th input sequence element. Input is terminated by a sequence of length n = 0. This sequence must not be processed.

Output

For every input sequence, your program prints a single line containing an integer number op, the minimum number of swap operations necessary to sort the given input sequence.

Sample Input

5
9
1
0
5
4
3
1
2
3
0

Sample Output

6
0

题意：给出一串数，求出使用快排变成从小到大排序的最小交换次数。

题解：根据快排加找规律，可以看出就是求这一串数的逆序数，然后就用树状数组写了。因为数据规模大，需要离散化一下，就是去重，重复的数用相同的数表示，所以需要排序。具体看代码。

import java.util.*;
public class Main {
	static Scanner cin = new Scanner(System.in);
	static int n;
	static int [] a;
	static int [] c;
	static int lowbit(int x) {
		return x&(-x);
	}
	static class Node implements Comparable<Node>{
		int val;
		int index;
		public int compareTo(Node w) {
			return this.val-w.val;
		}
	}//离散化，因为数据大
	static int query(int x) { //1-x求和
		int sum=0;
		while(x!=0) {
			sum+=a[x];
			x-=lowbit(x);
		}
		return sum;
	}
	static void add(int x,int v) {
		while(x<=n) {
			a[x]+=v;
			x+=lowbit(x);
		}
	}
	public static void main(String[] args){
		while(cin.hasNext()) {
			long sum=0;
			n = cin.nextInt();
			Node[] node=new Node[n];
			if(n==0) break;
			a = new int [n+5];
			c = new int [n+5];
			for (int i = 0; i < n;i++) {
				node[i] = new Node();
				node[i].val=cin.nextInt();
				node[i].index=i+1;
			}
			Arrays.sort(node);//排序
			int id=1;
			//Arrays.fill(c,0);
			c[node[0].index]=id;	
			for(int i = 1;i < n;i++){//离散化
				if(node[i].val!=node[i-1].val)
					c[node[i].index]=++id;
				else
					c[node[i].index]=id;
			}
			for(int i = 1;i <= n;i++){
				add(c[i],1);//单点更新
				sum+=i-query(c[i]);//求 逆序数
			}
			System.out.println(sum);
		}
	}
}