leetcode-509-easy

Fibonacci Number

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: 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.
Constraints:

0 <= n <= 30

思路一: 动态规划

public int fib(int n) {
    if (n < 2) return n;
    
    int n1 = 0;
    int n2 = 1;
    int result = 0;
    for (int i = 2; i < n; i++) {
        result = n2 + n1;
        n1 = n2;
        n2 = result;
    }

    return result;
}