1137. N-th Tribonacci Number
Easy
The Tribonacci sequence Tn is defined as follows:
T0 = 0, T1 = 1, T2 = 1, and Tn+3 = Tn + Tn+1 + Tn+2 for n >= 0.
Given n, return the value of Tn.
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];
}
}