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Formatted question description: https://leetcode.ca/all/509.html

# 509. Fibonacci Number (Easy)

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.

Related Topics:
Array

Similar Questions:

## Solution 1.

// OJ: https://leetcode.com/problems/fibonacci-number/
// Time: O(N)
// Space: O(1)
class Solution {
public:
int fib(int N) {
if (!N) return 0;
if (N == 1) return 1;
int prev = 0, cur = 1;
while (--N > 0) {
int tmp = prev + cur;
prev = cur;
cur = tmp;
}
return cur;
}
};


Java

• class Solution {
public int fib(int N) {
if (N <= 1)
return N;
int prev2 = 0, prev1 = 1;
int num = 1;
int index = 2;
while (index <= N) {
num = prev2 + prev1;
prev2 = prev1;
prev1 = num;
index++;
}
return num;
}
}

• // OJ: https://leetcode.com/problems/fibonacci-number/
// Time: O(N)
// Space: O(1)
class Solution {
public:
int fib(int n) {
if (n <= 1) return n;
int a = 0, b = 1;
while (--n > 0) {
a += b;
swap(a, b);
}
return b;
}
};

• class Solution:
def fib(self, n: int) -> int:
a, b = 0, 1
for _ in range(n):
a, b = b, a + b
return a