POJ2081 2081 Recaman's Sequence

#include<stdio.h>

const int maxn=500001;
int a[maxn];
bool vis[1<<30];
int main()
{
int k,i;
a[0]=0;vis[0]=1;
for(i=1;i<maxn;i++)
if(a[i-1]-i>0&&!vis[a[i-1]-i])
{
a[i]=a[i-1]-i;
vis[a[i]]=1;
}
else
{
a[i]=a[i-1]+i;
vis[a[i]]=1;
}
while(1)
{
scanf("%d",&k);
if(k==-1)break;
printf("%d\n",a[k]);
}
return 0;
}