【二分答案】【字符串哈希】bzoj2084 [Poi2010]Antisymmetry

#include<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
#define N 500001
typedef unsigned long long ull;
const ull seed=3;
ull seeds[N],pre[N],rsuf[N],ans;
int n;
char s[N],s2[N];
ull Get(int l,int r){return pre[r]-pre[l-1]*seeds[r-l+1];}
ull Get2(int l,int r){return rsuf[l]-rsuf[r+1]*seeds[r-l+1];}
bool check(int x,int len){return Get(x-len+1,x+len)==Get2(x-len+1,x+len);}
void solve(int x)
{
	int l=0,r=min(x+1,n-x-1);
	while(r>l)
	  {
	  	int mid=(l+r+1>>1);
	  	if(check(x,mid)) l=mid;
	  	else r=mid-1;
	  }
	ans+=(ull)l;
}
int main()
{
	scanf("%d%s",&n,s);
	seeds[0]=1;
	for(int i=1;i<=n;++i) seeds[i]=seeds[i-1]*seed;
	for(int i=0;i<n;++i) pre[i]=pre[i-1]*seed+(s[i]-‘0‘);
	for(int i=0;i<n;++i) s2[i]=(s[i]==‘1‘?‘0‘:‘1‘);
	for(int i=n-1;i>=0;--i) rsuf[i]=rsuf[i+1]*seed+(s2[i]-‘0‘);
	for(int i=0;i<n-1;++i)
	  solve(i);
	cout<<ans<<endl;
	return 0;
}