Poj2229--Sumsets（递推）

#include <cmath>
#include <cstdio> 
const int  MOD = 1e9;
const int N = 1000001;
int num[N];
void Sieve(){
    num[1] = 1;
    num[2] = 2;
    num[3] = 2;
    for(int i = 4; i <= N; i++){
        if(i & 1)
            num[i] = num[i-1];
        else
            num[i] = (num[i-2]%MOD + num[i/2]%MOD)%MOD;
    }
}
int main(){
    Sieve();
    int n;
    while(scanf("%d", &n) != EOF)
        printf("%d\n", num[n]);
    return 0;
}