# Fibonacci Number Problem

## Description

LeetCode Problem 509.

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,

``````1
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F(0) = 0, F(1) = 1
F(n) = F(n - 1) + F(n - 2), for n > 1.
``````

Given n, calculate F(n).

Example 1:

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

Example 2:

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

Example 3:

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

Constraints:

• 0 <= n <= 30

## Sample C++ Code

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class Solution {
public:
int fib(int N) {
if (N == 0)
return 0;
if (N == 1)
return 1;
return fib(N-1) + fib(N-2);
}
};
``````
``````1
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class Solution {
public:
int fib(int N) {
int dp = {0, 1};

for (int i = 2; i <= N; i++)
dp[i % 2] = dp + dp;

return dp[N % 2];
}
};
``````