LeetCode 509. Fibonacci Number
原题链接在这里：https://leetcode.com/problems/fibonacci-number/
题目：
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.
题解：
Having 2 numbers adding up to 3rd number and update 1st and 2nd number.
Time Complexity: O(N).
Space: O(1).
AC Java:
1 class Solution { 2 public int fib(int N) { 3 if(N == 0 || N == 1){ 4 return N; 5 } 6 7 int first = 0; 8 int second = 1; 9 for(int i = 1; i < N; i++){ 10 int third = first + second; 11 first = second; 12 second = third; 13 } 14 15 return second; 16 } 17 }
