斐波那契通项公式

O(1)通项公式解法：

public class Solution {
    public int Fibonacci(int n) {
        double a = Math.sqrt(5.0);
        double temp1 = (1.0 + a)/2.0;
        double temp2 = (1.0 - a)/2.0;
        return (int)(1.0/a*(Math.pow(temp1, n) - Math.pow(temp2, n)));
    }
}