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N-th Tribonacci Number - Recursion - Easy - LeetCode

N-th Tribonacci Number - Recursion - Easy - LeetCode

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.

public class Solution {
    Dictionary<int,int> T = new Dictionary<int,int>();
    public int Tribonacci(int n) {
        if(T.ContainsKey(n)){
            return T[n];
        }
        if(n==0){
            return 0;
        }
        
        if(n==1 || n==2){
            return 1;
        }
        
        int res = Tribonacci(n-1)+Tribonacci(n-2)+Tribonacci(n-3);
        T[n]=res;
        return res;
    }
}

Time Complexity: O(n)

Space Complexity: O(n)