README.md

1137. N-th Tribonacci Number

The Tribonacci sequence T_n is defined as follows:

T₀ = 0, T₁ = 1, T₂ = 1, and T_n+3 = T_n + T_n+1 + T_n+2 for n >= 0.

Given n, return the value of T_n.

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 <= 231 - 1.

Solutions (Rust)

1. Recursion

implSolution{pubfntribonacci(n:i32) -> i32{fnhelper(n:usize) -> Vec<i32>{match n {0 => vec![0],1 => vec![0,1],2 => vec![0,1,1], _ => {letmut v = helper(n - 1);let t_n = v[n - 3] + v[n - 2] + v[n - 1]; v.push(t_n); v },}}let n = n asusize;helper(n)[n]}}

2. Iteration

implSolution{pubfntribonacci(n:i32) -> i32{match n {0 => 0,1 | 2 => 1, _ => {letmut t = (0,1,1);for i in3..=n { t = (t.1, t.2, t.0 + t.1 + t.2);} t.2},}}}