树状数组之逆序数

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
using namespace std;

const int N = 500005;

struct Node
{
int val;//存储的是每个值，用以排序
int pos;//存储的是每个值出事序列的下标，保证在排序之后仍保留原本的顺序
};

Node node
;
int c
, reflect
, n;

bool cmp(const Node& a, const Node& b)
{
return a.val < b.val;
}

int lowbit(int x)
{
return x & (-x);
}

void update(int x)
{
while (x <= n)
{
c[x] += 1;
x += lowbit(x);
}
}

int getsum(int x)
{
int sum = 0;
while (x > 0)
{
sum += c[x];
x -= lowbit(x);
}
return sum;
}

int main()
{
while (scanf("%d", &n) != EOF && n)
{
for (int i = 1; i <= n; ++i)
{
scanf("%d", &node[i].val);
node[i].pos = i;
}
sort(node + 1, node + n + 1, cmp);   //排序
for (int i = 1; i <= n; ++i) reflect[node[i].pos] = i;   //离散化
for (int i = 1; i <= n; ++i) c[i] = 0;   //初始化树状数组
long long ans = 0;
for (int i = 1; i <= n; ++i)
{
update(reflect[i]);
int b= i - getsum(reflect[i]);
ans+=b;
}
printf("%lld\n", ans);
}
return 0;
}