1137.n-th-tribonacci-number.cpp

class Solution {
public:
    int tribonacci(int n) {
        int f[3]={0,1,1};
        for(int i=3;i<=n;i++)
            f[i%3]+=f[(i+1)%3]+f[(i+2)%3];
        return f[n%3];
    }
};