原题:
Problem Description

Ultra-QuickSort produces the output
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n和n个数,每次只能交换相邻两个元素,求多少次交换后能使这些数升序排列。
思路:典型的求逆序数的问题。由于数组元素可能过大,所以需要用到树状数组离散化。
源代码:
#include <iostream>
#include <stdio.h>
#include <cstring>
#include <algorithm>
#define MAX 500010
using namespace std;
int C[MAX];
int b[MAX];//备用数组,用于储存初始数组重新赋值后的结果
int lowbit(int x)
{
return x &(-x);
}
int getsum(int x)
{
int res = 0;
while (x > 0)
{
res += C[x];
x -= lowbit(x);
}
return res;
}
void add(int x,int v)
{
while (x < MAX)
{
C[x] += v;
x += lowbit(x);
}
}
struct Point//用于储存该点的值及其初始位置
{
int v;
int id;
}point[MAX];
bool cmp(Point a,Point b)
{
return a.v < b.v;
}
int main()
{
int n;
while (scanf("%d",&n)!=EOF)
{
if (n == 0)
break;
for (int i = 1; i <= n; i++)
{
scanf("%d",&point[i].v);
point[i].id = i;
}
sort (point+1,point + n + 1,cmp);
memset(b,0,sizeof(b));
memset(C,0,sizeof(C));//初始值一定为0
b[point[1].id] = 1;
for (int i = 2; i <= n; i++)//数组离散化
{
if (point[i].v == point[i-1].v)
b[point[i].id] = b[point[i-1].id];
else
b[point[i].id] = i;
}
//for (int i = 1; i <= n; i++)
// cout << b[i] << " ";
//cout << endl;
long long ans = 0;
for (int i = 1; i <= n; i++)//求逆序数
{
add(b[i],1);
ans += getsum(n) - getsum(b[i]);
// cout << ans << endl;
}
//for (int i = 1; i <= n; i++)
// cout << C[i] << " ";
//cout << endl;
printf("%lld\n",ans);
}
return 0;
}