1137. N-th Tribonacci Number

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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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Intuition

Similar to Fibonacci series
F(N) = F(N - 1) + F(N - 2) + F(N - 3)

We just need last three entries, no need for full DP table
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Time - O(n)
Space - O(1)
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Related problems

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