PAT L2-014. 列车调度

#include<map>
#include<set>
#include<ctime>
#include<cmath>
#include<queue>
#include<string>
#include<stack>
#include<vector>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<functional>
using namespace std;

int n,a[100010],c[100010];

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

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

int get(int x)
{
int p=x,res=0;
while(p)
{
res=res+c[p];
p=p-lowbit(p);
}
return res;
}

int main()
{
scanf("%d",&n); int ans=0;
for(int i=1;i<=n;i++)
{
scanf("%d",&a[i]);
int sum = get(n)-get(a[i]);
if(sum==0)
{
update(a[i],1);
ans++;
continue;
}

int L=a[i], R=n, pos=-1;
while(L<=R)
{
int mid = (L+R)/2;
if(get(n)-get(mid-1)==sum) L = mid+1,pos=mid;
else R = mid-1;
}

update(pos,-1);
update(a[i],1);
}

printf("%d\n",ans);
return 0;
}