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,

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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:

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

Example 2:

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

Example 3:

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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);
    }
};
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class Solution {
public:
    int fib(int N) {
        int dp[2] = {0, 1};

        for (int i = 2; i <= N; i++)
           dp[i % 2] = dp[0] + dp[1];

        return dp[N % 2];
    }
};




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