分块算法学习记录

分块算法其实很神奇，就是把一个数列划分为sqrtN块，然后暴力就会得到意想不到的速度。

HNOI 2010 BZOJ 2002 弹飞绵羊

其实分块的真的没什么可讲的code：

#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstring>
using namespace std;
struct hp{
	int step,next;
}a[200001];
int n,m;
int ki[200001];
int block[501];
int main()
{
	int i,per,t=0,opt,ans,x,k,j,dn;
	scanf("%d",&n); per=sqrt(n);
	for (i=1;i<=n;++i)
	  scanf("%d",&ki[i]);
	block[0]=1; block[1]=n; t=0;
	for (i=n;i>=1;--i)
	  {
	    t++;
	    if (i+ki[i]>block[block[0]])
	      {
	        a[i].step=1;
	        a[i].next=i+ki[i];
	      }
	    else
	      {
	        a[i].step=a[i+ki[i]].step+1;
	        a[i].next=a[i+ki[i]].next;
	      }
		if (t==per)
		  {
		    t=0;
			block[++block[0]]=i-1;
	      }
	  }
	if (t==0) block[0]--;
	for (i=1;i<=block[0]/2;++i)
	  swap(block[i],block[block[0]-i+1]);
	scanf("%d",&m);
	for (i=1;i<=m;++i)
	  {
	    scanf("%d",&opt);
	    if (opt==1)
	      {
	      	ans=0;
	        scanf("%d",&x); x++;
	        while (x<=n)
	          {ans+=a[x].step; x=a[x].next; }
	        printf("%d\n",ans);
	      }
	    if (opt==2)
	      {
	        scanf("%d%d",&x,&k); x++;
			t=upper_bound(block+1,block+block[0]+1,x)-block-1;
			if (block[t]!=x) t++; 
			if (t>1) dn=block[t-1]+1; 
			else dn=1;
			ki[x]=k;
			for (j=x;j>=dn;--j)
			  {
			    if (j+ki[j]<=block[t])
			      {a[j].step=a[j+ki[j]].step+1; a[j].next=a[j+ki[j]].next;}
			    else
			      {a[j].step=1; a[j].next=j+ki[j];}
			  }  
	      }
	  }
}