1137. 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.

Rust Solution

struct Solution;

impl Solution {
    fn tribonacci(n: i32) -> i32 {
        let n = n as usize;
        let mut a: Vec<i32> = vec![0; 38];
        a[0] = 0;
        a[1] = 1;
        a[2] = 1;
        for i in 3..=n {
            a[i] = a[i - 1] + a[i - 2] + a[i - 3];
        }
        a[n]
    }
}

#[test]
fn test() {
    assert_eq!(Solution::tribonacci(25), 1_389_537);
}

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