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.
Example
Given 1, return 0
Given 2, return 1
Given 10, return 34
题意
查找斐波纳契数列中第 N 个数。
所谓的斐波纳契数列是指：
- 前2个数是 0 和 1 。
- 第 i 个数是第 i-1 个数和第i-2 个数的和。
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 };
递归会超时