1137. N-th Tribonacci Number
https://leetcode.com/problems/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.
---
Intuition
Similar to Fibonacci series
F(N) = F(N - 1) + F(N - 2) + F(N - 3)
We just need last three entries, no need for full DP table
----
Time - O(n)
Space - O(1)
---
Related problems
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| class Solution { | |
| public int tribonacci(int n) { | |
| if (n < 2) { | |
| return n; | |
| } | |
| if (n == 2) { | |
| return 1; | |
| } | |
| int first = 0, second = 1, third = 1, fourth = 0; | |
| for (int i = 3; i <= n; i++) { | |
| fourth = first + second + third; | |
| first = second; | |
| second = third; | |
| third = fourth; | |
| } | |
| return fourth; | |
| } | |
| } |
