POJ 2081

 1 #include <iostream>
 2 #define MAXN  500005
 3 using namespace std;
 4 
 5 //unsigned _m[MAXN];
 6 bool mark[MAXN*10];
 7 
 8 int main()
 9 {
10     //freopen("acm.acm","r",stdin);
11 //    _m[0] = 0;
12 
13     int i;
14     int num;
15 
16     int p_1;
17     int p_2;
18     
19     while(cin>>num,num != -1)
20     {
21         p_1 = p_2 = 0;
22         memset(mark,0,sizeof(mark));
23         for(i = 1; i <= num; ++ i)
24         {
25             
26             if(p_1 - i > 0 && !mark[p_1 - i])
27             {
28                 p_2 = p_1 - i;
29                 mark[p_2] = true;
30             }
31             else
32             {
33                 p_2 = p_1 + i;
34                 mark[p_2] = true;
35             }
36             p_1 = p_2;
37         }
38         cout<<p_2<<endl;
39     }
40 }