Description Link to heading

509.fibonacci number

Solution Link to heading

$a_n = a_{n - 1} + a_{n - 2}$.

Based on the recursive relationship, we can easily write traversal code using for loop.

Code Link to heading 

class Solution {
public:
    int fib(int N) {
        if (N <= 1) return N;
        int dp[2];
        dp[0] = 0;
        dp[1] = 1;
        for (int i = 2; i <= N; i++) {
            int sum = dp[0] + dp[1];
            dp[0] = dp[1];
            dp[1] = sum;
        }
        return dp[1];
    }
};