1137.cpp

class Solution {
public:
    int tribonacci(int n) {
        
        if(n < 2)
            return n;
        
        else if(n == 2)
            return 1;
        
        else{
            int first = 0, second = 1, third = 1;
            
            int sum = 0;
            
            for(int i=3; i<=n; i++){
                sum = first + second + third;
                first = second;
                second = third;
                third = sum;
            }
            
            return sum;
        }
    }
};