add leetcode problem-cn part1

算法题（国内版）/problem (English)/第 N 个泰波那契数(English) [n-th-tribonacci-number].html

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+<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>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>