剑指 Offer 10- I. 斐波那契数列

剑指 Offer 10- I. 斐波那契数列

方法一

class Solution {
    int mod = (int) 1e9 + 7;
    
    public int fib(int n) {
        if(n <= 1) return n;
        int[] dp = new int[n + 1];
        dp[1] = 1;

        for(int i = 2; i <= n; i++){
            dp[i] = (dp[i - 1] + dp[i - 2]) % mod;
        }

        return dp[n];
    }
}

方法二

对方法一进行空间优化。

class Solution {
    int mod = (int) 1e9 + 7;
    
    public int fib(int n) {
        if(n <= 1) return n;
        int a = 0, b = 1;

        for(int i = 2; i <= n; i++){
            int c = (a + b) % mod;
            a = b;
            b = c;
        }

        return b;
    }
}