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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

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Solution 1.

• Java
• C++
• Python
• Go
• TypeScript
• Javascript
• Php
• RenderScript

• 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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