Math - N-th Tribonacci Number

T(0)=0, T(1)=1, T(2)=1, T(n+3)=T(n)+T(n+1)+T(n+2). Given n, return T(n).

Input: n=4 → Output: 4

Input: n=25 → Output: 1389537

Iterative DP with three variables.

Base cases: T(0)=0, T(1)=1, T(2)=1.
For i from 3 to n: T = T0+T1+T2; shift variables.

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 t=a+b+c;a=b;b=c;c=t;} return c; } }

Time Complexity: O(N)
Space Complexity: O(1)