1137

1137. N-th Tribonacci Number (Easy)

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 <= 2^31 - 1.

Companies:
Amazon

Related Topics:
Math, Dynamic Programming, Memoization

Similar Questions:

• Climbing Stairs (Easy)
• Fibonacci Number (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/n-th-tribonacci-number/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    int tribonacci(int n) {
        if (n == 0) return 0;
        if (n <= 2) return 1;
        int a = 0, b = 1, c = 1;
        for (int i = 3; i <= n; ++i) {
            int next = a + b + c;
            a = b;
            b = c;
            c = next;
        }
        return c;
    }
};