1137. N-th Tribonacci Number

Easy

The Tribonacci sequence T_n is defined as follows:

T₀ = 0, T₁ = 1, T₂ = 1, and T_n+3 = T_n + Tn+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 <= 2^31 - 1.

Solution:

class Solution {

    /**
     * @param Integer $n
     * @return Integer
     */
    function tribonacci($n) {
        $dp = [0, 1, 1];

        for ($i = 3; $i < $n + 1; $i++) {
        $dp[$i % 3] = array_sum($dp);
        }

        return $dp[$n % 3];
    }
}

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