### 我的答案

class Solution {
public int fib(int n) {
int x =f(n);
return x%1000000007;

}

public int f(int n){
if(n<0) return -1;
if(n==0) return 0;
if(n==1) return 1;
else return f(n-1)+f(n-2);

}
}


(°д°)

class Solution {
int a=0,b=1;
int res;
public int fib(int n) {

if(n<0) return -1;
else if(n==0) return 0;
else if(n==1) return 1;
else for(int i=1;i<n;i++){
res = a+b;
a=b;
b=res%1000000007;

}

return res%1000000007;

}
}


### 优秀答案

O(1)复杂度

int f[]={0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181,6765,10946,17711,28657,46368,75025,121393,196418,317811,514229,832040,1346269,2178309,3524578,5702887,9227465,14930352,24157817,39088169,63245986,102334155,165580141,267914296,433494437,701408733,134903163,836311896,971215059,807526948,778742000,586268941,365010934,951279875,316290802,267570670,583861472,851432142,435293607,286725742,722019349,8745084,730764433,739509517,470273943,209783453,680057396,889840849,569898238,459739080,29637311,489376391,519013702,8390086,527403788,535793874,63197655,598991529,662189184,261180706,923369890,184550589,107920472,292471061,400391533,692862594,93254120,786116714,879370834,665487541,544858368,210345902,755204270,965550172,720754435,686304600,407059028,93363621,500422649,593786270,94208912,687995182};
int fib(int n){
return f[n];
}