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509.fibonacci-number.ts
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509.fibonacci-number.ts
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/*
* @lc app=leetcode id=509 lang=typescript
*
* [509] Fibonacci Number
*
* https://leetcode.com/problems/fibonacci-number/description/
*
* algorithms
* Easy (68.32%)
* Likes: 3803
* Dislikes: 270
* Total Accepted: 833.6K
* Total Submissions: 1.2M
* Testcase Example: '2'
*
* 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: n = 2
* Output: 1
* Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.
*
*
* Example 2:
*
*
* Input: n = 3
* Output: 2
* Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.
*
*
* Example 3:
*
*
* Input: n = 4
* Output: 3
* Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.
*
*
*
* Constraints:
*
*
* 0 <= n <= 30
*
*
*/
// @lc code=start
function fib(n: number): number {
if (n <= 1) return n;
let pre1 = 1,
pre2 = 0;
for (let i = 2; i <= n; i++) {
const temp = pre2;
pre2 = pre1;
pre1 = temp + pre1;
}
return pre1;
}
// tail recursive
// function fib(n: number): number {
// if (n <= 1) return n;
// return fibTail(n, 1, 0);
// }
// function fibTail(n: number, acc1: number, acc2) {
// if (n > 2) return fibTail(n - 1, acc1 + acc2, acc1);
// return acc1 + acc2;
// }
// @lc code=end