1137. 第 N 个泰波那契数/C++

int tribonacci(int n) {
    if(n<0)
        return -1;
    else if(n==0)
        return 0;
    else if(n==1 || n==2)
        return 1;
    else{
        int n0=0,n1=1,n2=1,n3;
        for(int i=3;i<=n;++i){
            n3=n0+n1+n2;
            n0=n1;
            n1=n2;
            n2=n3;
        }
        return n3;
    } 
}