POJ 2081 Recaman's Sequence

 1 //oimonster
 2 #include<cstdio>
 3 #include<cstdlib>
 4 #include<iostream>
 5 using namespace std;
 6 int a[500001],b[10000000]={0};
 7 int main(){
 8     int i,j,n,m;
 9     a[0]=0;
10     b[0]=1;
11     for(i=1;i<=500000;i++){
12         m=a[i-1]-i;
13         if((m<=0)||(b[m]==1)){
14             a[i]=a[i-1]+i;
15             b[a[i]]=1;
16         }
17         else{
18             a[i]=m;
19             b[m]=1;
20         }
21     }
22     scanf("%d",&n);
23     while(n!=-1){
24         printf("%d\n",a[n]);
25         scanf("%d",&n);
26     }
27     return 0;
28 }