1137. N-th Tribonacci Number.md

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.

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Solution

• mine

  • Java

    Runtime: 0 ms, faster than 100.00%, Memory Usage: 35.7 MB, less than 100.00% of Java online submissions

    //O(N)time O(1)space
    public int tribonacci(int n) {
        if(n == 0){
            return 0;
        }
        if(n < 3){
            return 1;
        }
        int r2 = 1, r1 = 1 , r0 = 0;
        for(int i = 3; i <= n; i++){
            int t = r2 + r1 + r0;
            r0 = r1;
            r1 = r2;
            r2 = t;
        }
        return r2;
    }
    

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