題意：輸入一個(gè)整數(shù)n，求斐波那契數(shù)列的第n項(xiàng)

算法：遞推

思路：用兩個(gè)變量滾動(dòng)往后計(jì)算，a表示第n - 1項(xiàng)，b表示第n項(xiàng)，c表示第n + 1項(xiàng)

時(shí)間復(fù)雜度：O(n)

int Fibonacci(int n) {
    if(n == 0) return 0;
    if(n == 1) return 1;
    
    int a = 0, b = 1;
    int c = 0;
    for(int i = 2;i <= n;i ++)
    {
        c = a + b;
        a = b;
        b = c;
    }
    return c;
}