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 ...

Notice

The Nth fibonacci number won‘t exceed the max value of signed 32-bit integer in the test cases.

Given 1, return 0

Given 2, return 1

Given 10, return 34

題意

查找斐波納契數列中第 N 個數。

所謂的斐波納契數列是指：

• 前2個數是 0 和 1 。
• 第 i 個數是第 i-1 個數和第i-2 個數的和。

解法一：

 1 class Solution {
 2 public:
 3     /*
 4      * @param n: an integer
 5      * @return: an ineger f(n)
 6      */
 7
 
     int fibonacci(int n) {
 8         int a, b;
 9         a = 0;
10         b = 1;
11         
12         for (int i = 1; i < n; i++) {
13             int c = a + b;
14             a = b;
15             b = c;
16         }
17         return a;
18     }
19 };

解法二：

 1 class Solution {
 
 2 public:
 3     /*
 4      * @param n: an integer
 5      * @return: an ineger f(n)
 6      */
 7     int fibonacci(int n) {
 8         if (n == 1) {
 9             return 0;
10         } else if (n == 2) {
11             return 1;
12         }
13         
14         return fibonacci(n - 1) + fibonacci(n - 2);
15                
16     }
17 };

遞歸會超時

366. Fibonacci【Naive】