【剑指offer】斐波那契数列非递归求解第N项

public class Solution {
    public int Fibonacci(int n) {
       //错误输入处理
       if(n<0) return -1;
       int pre = 1;
       int result = 0;
       for(int i=0; i<n; i++){
           //计算第i项
           result += pre;
           //将pre往后更新一项
           pre = result - pre;
       }

        return result;
    }
}