1137. N-th Tribonacci Number.cpp

class Solution {
public:
    int tribonacci(int n) {
        vector<int> a(38);
        a[0] = 0;
        a[1] = a[2] = 1;
        for (int i=3; i<=n; ++i)
            a[i] = a[i-1] + a[i-2] + a[i-3];
        return a[n];
    }
};