1137. N-th Tribonacci Number #

Problem #

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.

Problem Summary #

The Tribonacci sequence Tn is defined as follows: 

T0 = 0, T1 = 1, T2 = 1, and under the condition n >= 0, Tn+3 = Tn + Tn+1 + Tn+2

Given the integer n, return the value of the n-th Tribonacci number Tn.

Note:

• 0 <= n <= 37
• The answer is guaranteed to be a 32-bit integer, i.e. answer <= 2^31 - 1.

Solution Approach #

• Find the n-th number in the Tribonacci sequence.
• Easy problem; just compute according to the definition given in the problem.

Code #

package leetcode

func tribonacci(n int) int {
	if n < 2 {
		return n
	}
	trib, prev, prev2 := 1, 1, 0
	for n > 2 {
		trib, prev, prev2 = trib+prev+prev2, trib, prev
		n--
	}
	return trib
}