leetcode 1137 - N-th Tribonacci Number

https://leetcode.com/problems/n-th-tribonacci-number/

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.

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class Solution {
public:
int tribonacci(int n) {
int buf[3] = {0, 1, 1};

if (n <= 2) return buf[n];

int tmp;

for (int i = 3; i <= n; i++) {
tmp = buf[0] + buf[1] + buf[2];

buf[0] = buf[1];
buf[1] = buf[2];
buf[2] = tmp;
}

return tmp;
}
};