POJ 2081 Recaman's Sequence

//POJ 2081  AC

#include <iostream>

using namespace std;

int a[500010];

bool flag[3500000]={false};//如果也开个500010  运行时错误

void init(){

int i,t;

a[0]=0;

for(i=1;i<=500000;i++){

t=a[i-1]-i;

if(t>0&&!flag[t]){

a[i]=t;

}

else{

a[i]=a[i-1]+i;

}

flag[a[i]]=true;

}

}

int main(){

int k;

init();

while(cin>>k){

if(k==-1) break;

cout<<a[k]<<endl;

}

return 0;

}