1137. N-th Tribonacci Number

The Tribonacci sequenceT n T_nTn​is defined as follows:

T 0 = 0 , T 1 = 1 , T 2 = 1 , a n d T n + 3 = T n + T n + 1 + T n + 2 f o r n > = 0. T_0 = 0, T_1 = 1, T_2 = 1, and T_{n+3} = T_n + T_{n+1} + T_{n+2} for n >= 0.T0​=0,T1​=1,T2​=1,andTn+3​=Tn​+Tn+1​+Tn+2​forn>=0.

Given n, return the value ofT n T_nTn​.

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.

From: LeetCode
Link: 1137. N-th Tribonacci Number

Solution:

Ideas:

Iteratively calculate Tribonacci numbers
using the previous 3 values only.

Code:

inttribonacci(intn){if(n==0)return0;if(n==1||n==2)return1;inta=0,b=1,c=1;for(inti=3;i<=n;i++){intd=a+b+c;a=b;b=c;c=d;}returnc;}