归并排序逆序对+快读

归并排序模版

使用前须将cnt置为0

void merge_sort(int *A,int x,int y,int *T,int &cnt)
{
if(y-x>1)
{
int m = x+(y-x)/2;
int p = x,q = m,i = x;
merge_sort(A,x,m,T,cnt);
merge_sort(A,m,y,T,cnt);
while(p<m || q<y)
{
if(q >= y || (p<m && A
<=A[q])) T[i++] = A[p++];
else {T[i++] = A[q++];cnt += m - p; }
}
for(int i = x;i < y;i++)A[i] = T[i];
}
}

逆序对解释：

[p]因为数组是从小到大复制的，从A[q]A[q]A[q]复制到T[i]T[i]T[i]时，左边还没来得及复制的那一些一些就是左边所有比A[q]A[q]A[q]大的数，此时只需要在**左边加上m−pm-pm−p**即可。

###快读模版

inline int read()
{
int x = 0,f = 1;char c = getchar();
while(!isdigit(c)){if(c=='-')f=-1;else f=1;c=getchar();}
while(isdigit(c)){x=(x<<1)+(x<<3)+(c^48);c=getchar();}
return x*f;
}
inline void write_num(int x)
{
if(x>9)write_num(x/10);
putchar((x%10)+48);
}
inline void write(int x)
{
if(x < 0)putchar('-'),x=-x;
write_num(x);
}

test

####以下便是完整的代码~~，欢迎拍砖~~

#include<bits/stdc++.h>
using namespace std;
const int maxn = 1e5+10;
int a[maxn],t[maxn];
void merge_sort(int *A,int x,int y,int *T,int &cnt)
{
if(y-x>1)
{
int m = x+(y-x)/2;
int p = x,q = m,i = x;
merge_sort(A,x,m,T,cnt);
merge_sort(A,m,y,T,cnt);
while(p<m || q<y)
{
if(q >= y || (p<m && A[p]<=A[q])) T[i++] = A[p++];
else {T[i++] = A[q++];cnt += m - p; }
}
for(int i = x;i < y;i++)A[i] = T[i];
}
}
inline int read()
{
int x = 0,f = 1;char c = getchar();
while(!isdigit(c)){if(c=='-')f=-1;else f=1;c=getchar();}
while(isdigit(c)){x=(x<<1)+(x<<3)+(c^48);c=getchar();}
return x*f;
}
inline void write_num(int x)
{
if(x>9)write_num(x/10);
putchar((x%10)+48);
}
inline void write(int x)
{
if(x < 0)putchar('-'),x=-x;
write_num(x);
}int main()
{
int n = read(),cnt = 0;
for(int i = 0;i < n;i++)
a[i] = read();
merge_sort(a,0,n,t,cnt);
write(cnt),putchar('\n');
for(int i = 0;i < n;i++)
write(a[i]),putchar(' ');
return 0;
}

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