N Th Tribonacci Number

Description Link to heading

The Tribonacci sequence T is defined as follows:

T = 0, T = 1, T = 1, and T = T + T + T for n >= 0.

Given n, return the value of T.

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.

Solution Link to heading

Dynamic programming, easy

Code Link to heading

class Solution {
public:
    int tribonacci(int n) {
        int num0 = 0, num1 = 1, num2 = 1;
        if (n == 0) {
            return num0;
        }
        if (n == 1) {
            return num1;
        }
        if (n == 2) {
            return num2;
        }
        for (int i = 2; i <= n; i++) {
            int tmp2 = num2;
            int tmp = num1;
            num2 += num0 + num1;
            num1 = tmp2;
            num0 = tmp;

        }
        return num1;
    }
};