时间复杂度O(n),空间复杂度O(1)解斐波那契数列

#include <stdio.h>
#include <iostream>
using namespace std;
long long fibs1(int in_iN) {
    if(in_iN < 0) {
        return -1;
    }else if(in_iN == 0) {
        return 0;
    }else if(in_iN == 1) {
        return 1;
    }
    long long t_i1 = 0;
    long long t_i2 = 1;
    long long t_iValue = 0;
    int t_k = 2;
    while(t_k <= in_iN) {
        t_iValue = t_i1 + t_i2;
        t_i1 = t_i2;
        t_i2 = t_iValue;
        t_k ++;
    }
    return t_iValue;
}

long long fibs2(int in_iN) {
    if(in_iN < 0) {
        return -1;
    }else if(in_iN == 0) {
        return 0;
    }else if(in_iN == 1) {
        return 1;
    }else {
        return fibs2(in_iN - 1) + fibs2(in_iN - 2);
    }
}
int main()
{
    int    in_iN = 40;
    cout << fibs1(in_iN) << endl;
    cout << fibs2(in_iN) << endl;
    cin >> in_iN;
    return 0;
}