1137. N-th Tribonacci Number #

题目 #

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.

题目大意 #

泰波那契序列 Tn 定义如下： 

T0 = 0, T1 = 1, T2 = 1, 且在 n >= 0 的条件下 Tn+3 = Tn + Tn+1 + Tn+2

给你整数 n，请返回第 n 个泰波那契数 Tn 的值。

提示：

• 0 <= n <= 37
• 答案保证是一个 32 位整数，即 answer <= 2^31 - 1。

解题思路 #

• 求泰波那契数列中的第 n 个数。
• 简单题，按照题意定义计算即可。

代码 #

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
}