poj 2081 Recaman's Sequence

#include <iostream>            
using namespace std;
int list[500001];
bool flag[10000000];
int main()
{
int n,top=0;
    fill(flag,flag+sizeof(flag),1);
    list[0]=0;flag[0]=0;
while(scanf("%d",&n)&&n!=-1)
    {
while(top<n)
        {
if(list[top]-top-1>0&&flag[list[top]-top-1])
            {
                top++;
                list[top]=list[top-1]-top;
            }
else
            {
                top++;
                list[top]=list[top-1]+top;
            }
            flag[list[top]]=0;
        }
        printf("%d\n",list[n]);
    }
return 0;
}