力扣【动态规划】-简单-1137. N-th Tribonacci Number

The Tribonacci sequence Tn is defined as follows:

T0 = 0, T1 = 1, T2 = 1, and Tn+3 = Tn + Tn+1 + Tn+2 for n >= 0.

Given n, return the value of Tn.

Example 1:

Input: n = 4
Output: 4
Explanation:
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4
Example 2:

Input: n = 25
Output: 1389537

Constraints:

0 <= n <= 37
The answer is guaranteed to fit within a 32-bit integer, ie. answer <= 2^31 - 1.

class Solution {
public:
    int tribonacci(int n) {
        int f3=0, f2=1, f1 = 1;
        if(n==0){
            return 0;
        }
        if(n == 1){
            return 1;
        }
        if(n==2){
            return 1;
        }
        int ans = 0;

        for(int i=3; i<=n; i++){
            ans = f3+f2+f1;
            f3 = f2;
            f2 = f1;
            f1 = ans;
        }
        return ans;
    }
};