题目

大家都知道斐波那契数列，现在要求输入一个整数n，请你输出斐波那契数列的第n项（从0开始，第0项为0），n<=39 。

递归实现

public class Solution {
    public int Fibonacci(int n) {
        if( n <= 1){
            return n;
        }
        return Fibonacci(n-1) + Fibonacci(n-2);
    }
}

优化递归

public class Solution {
    public int Fibonacci(int n) {
        int ans[] = new int[40];
        ans[0] = 0;
        ans[1] = 1;
        for(int i=2;i<=n;i++){
            ans[i] = ans[i-1] + ans[i-2];
        }
        return ans[n];
    }
}

优化存储

public class Solution {
    public int Fibonacci(int n) {
        if(n < 2){
            return n;
        }
        int one = 0;
        int two = 1;
        int sum = 0;
        for(int i = 2; i <= n; i++ ){
            sum = one + two;
            one = two;
            two = sum;
        }
        return sum;
    }
}