Source

Find the Nth number in Fibonacci sequence.

A Fibonacci sequence is defined as follow:

The first two numbers are 0 and 1.
The i th number is the sum of i-1 th number and i-2 th number.
The first ten numbers in Fibonacci sequence is:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ...

Example
Given 1, return 0

Given 2, return 1

Given 10, return 34

Note
The Nth fibonacci number won't exceed the max value of
signed 32-bit integer in the test cases.

题解

斐波那契数列使用递归极其容易实现，其实使用非递归的方法也很容易，不断向前滚动即可。

Java

class Solution {
    /**
     * @param n: an integer
     * @return an integer f(n)
     */
    public int fibonacci(int n) {
        if (n < 0) return -1;
        if (n == 1) return 0;
        if (n == 2) return 1;

        int fn = 0, fn1 = 1, fn2 = 0;
        for (int i = 3; i <= n; i++) {
            fn = fn1 + fn2;
            fn2 = fn1;
            fn1 = fn;
        }

        return fn;
    }
}

源码分析

1. corner cases
2. 初始化 fn, fn1, fn2, 建立地推关系。
3. 注意 fn, fn2, fn1的递推顺序。

复杂度分析

遍历一次，时间复杂度为 O(n), 使用了两个额外变量，空间复杂度为 O(1).