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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.

  • 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;
        }
    }
    
    ############
    
    class Solution {
        public int fib(int n) {
            int a = 0, b = 1;
            while (n-- > 0) {
                int c = a + b;
                a = b;
                b = c;
            }
            return a;
        }
    }
    
  • // 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
    
    
    
  • func fib(n int) int {
    	a, b := 0, 1
    	for i := 0; i < n; i++ {
    		a, b = b, a+b
    	}
    	return a
    }
    
  • function fib(n: number): number {
        let a = 0;
        let b = 1;
        for (let i = 0; i < n; i++) {
            [a, b] = [a, a + b];
        }
        return a;
    }
    
    
  • /**
     * @param {number} n
     * @return {number}
     */
    var fib = function (n) {
        let a = 0;
        let b = 1;
        while (n--) {
            const c = a + b;
            a = b;
            b = c;
        }
        return a;
    };
    
    
  • class Solution {
        /**
         * @param Integer $n
         * @return Integer
         */
        function fib($n) {
            if ($n == 0 || $n == 1) {
                return $n;
            }
            $dp = [0, 1];
            for ($i = 2; $i <= $n; $i++) {
                $dp[$i] = $dp[$i - 2] + $dp[$i - 1];
            }
            return $dp[$n];
        }
    }
    
  • impl Solution {
        pub fn fib(n: i32) -> i32 {
            let mut a = 0;
            let mut b = 1;
            for _ in 0..n {
                let t = b;
                b = a + b;
                a = t
            }
            a
        }
    }
    
    

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