1013. Fibonacci Number

• Difficulty: Easy

• Topics: Array

• Similar Questions:

  • Climbing Stairs

  • Split Array into Fibonacci Sequence

  • Length of Longest Fibonacci Subsequence

  • N-th Tribonacci Number

Problem:

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.

Solutions:

class Solution {
public:
    int fib(int N) {
        if (N <= 1)  return N;

        int prev1 = 0;
        int prev2 = 1;
        for (int i = 2; i <= N; ++i) {
            int value = prev1 + prev2;
            prev1 = prev2;
            prev2 = value;
        }

        return prev2;
    }
};