Description：
The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,

F(0) = 0, F(1) = 1
F(N) = F(N - 1) + F(N - 2), for N > 1.

Given N, calculate F(N).

Example 1:

Input: 2
Output: 1
Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.

Example 2:

Input: 3
Output: 2
Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.

Example 3:

Input: 4
Output: 3
Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.

Note:

  • 0 ≤ N ≤ 30.

题意：计算斐波那契数列中的第N个的值F(N)；

解法：我们知道斐波那契数列有以下的计算公式
LeetCode-Fibonacci Number_Recursion
因此，我们只需要定义两个数f1和f2分别用于记录当前需要计算的数值和前一个计算的数值即可，最后，f1存储的就是我们所要求取的F(N)；

Java
class Solution {
    public int fib(int N) {
        if (N == 0) return 0;
        if (N == 1) return 1;
        int f1 = 0;
        int f2 = 1;
        for (int i = 2; i <= N; i++) {
            int temp = f2;
            f2 = f1 + f2;
            f1 = temp;
        }
        return f2;
    }
}