509. Fibonacci Number

Question

LeetCode Problem

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

Solutions

Use DP

int fib(int n) {
	vector<int> dp(n + 1);
	if (n == 0) return 0;
	if (n == 1) return 1;
	dp[0] = 0;
	dp[1] = 1;
	for (int i = 2; i <= n; ++i) {
		dp[i] = dp[i-1] + dp[i-2];
	}
	return dp[n];
}