LeetCode#1137. N-th Tribonacci Number

1137. N-th Tribonacci Number

The Tribonacci sequence T_n is defined as follows:
T₀ = 0, T₁ = 1, T₂ = 1, and T_n+3 = T_n + T_n+1 + T_n+2 for n >= 0.
Given n, return the value of T_n.

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 <= 231 - 1.

---

动态规划

class Solution {
    public int tribonacci(int n) {
        if (n == 0) {
            return 0;
        }
        if (n <= 2) {
            return 1;
        }
        int a = 0, b = 0, c = 1, d = 1;
        for (int i = 3; i <= n; ++i) {
            a = b;
            b = c;
            c = d;
            d = a + b + c;
        }
        return d;
    }
}