剑指offer（7）斐波那契数列

function Fibonacci(n)
{
    // write code here
    if(n <= 1){
        return n;
    }
    //用prepre指向上上项，pre指向上一项，now指向当前项
    var prepre = 0;
    var pre = 1;
    var now = 0;
    for(var i = 2;i <= n;i++){
        //从第二项开始，每一项等于上上项加上一项
        //得到当前项后，把pre赋值给prepre，now赋值给pre
        now  = pre + prepre;
        prepre = pre;
        pre = now;
    }
    return now;
}