509. Fibonacci Number

1. Question

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).

2. Examples

Example 1:

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

Example 2:

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

Example 3:

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

3. Constraints

• 0 <= n <= 30

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/fibonacci-number 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

类似于滑动窗口。

class Solution {
  public int fib(int n) {
    int a = 1;
    int b = 1;
    int c = 1;
    if (n < 2) {
      return n;
    }

    for (int i = 3; i <= n; i++) {
      c = a + b;
      a = b;
      b = c;
    }

    return c;
  }
}