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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.
// OJ: https://leetcode.com/problems/fibonacci-number/
// Time: O(N)
// Space: O(1)
class Solution {
public:
int fib(int N) {
if (!N) return 0;
if (N == 1) return 1;
int prev = 0, cur = 1;
while (--N > 0) {
int tmp = prev + cur;
prev = cur;
cur = tmp;
}
return cur;
}
};
Java
-
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; } }
-
// 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