<p>The Tribonacci sequence T<sub>n</sub> is defined as follows:&nbsp;</p>

<p>T<sub>0</sub> = 0, T<sub>1</sub> = 1, T<sub>2</sub> = 1, and T<sub>n+3</sub> = T<sub>n</sub> + T<sub>n+1</sub> + T<sub>n+2</sub> for n &gt;= 0.</p>

<p>Given <code>n</code>, return the value of T<sub>n</sub>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 4
<strong>Output:</strong> 4
<strong>Explanation:</strong>
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 25
<strong>Output:</strong> 1389537
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= n &lt;= 37</code></li>
	<li>The answer is guaranteed to fit within a 32-bit integer, ie. <code>answer &lt;= 2^31 - 1</code>.</li>
</ul>

class Solution {
public:
    int tribonacci(int n) {
        if (n <= 1)
            return n;

        int prev3 = 0, prev2 = 1, prev = 1;
        for (int i = 3; i <= n; i++) {
            int curr = prev3 + prev2 + prev;

            prev3 = prev2;
            prev2 = prev;
            prev = curr;
        }

        return prev;
    }
};
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