N-th Tribonacci Number

Author: anantkaushik

1137-n-th-tribonacci-number.py

@@ -23,13 +23,33 @@
 """
 class Solution:
  def tribonacci(self, n: int) -> int:
- if n == 0:
- return 0 
- elif n == 1 or n == 2:
+ if n <= 1:
+ return n
+ elif n == 2:
  return 1
- a = 0
- b = c = 1
- for i in range(2,n):
- ans = a+b+c
- a,b,c = b,c,ans
- return ans
+ 
+ first, second, third = 0, 1, 1
+ for _ in range(3, n+1):
+ first, second, third = second, third, first + second + third
+ 
+ return third
+
+
+class Solution1:
+ def tribonacci(self, n: int) -> int:
+ return self.helper(n, {0:0, 1:1, 2:1})
+ 
+ def helper(self, n, memo):
+ if n not in memo:
+ memo[n] = self.helper(n-1, memo) + self.helper(n-2, memo) + self.helper(n-3, memo)
+ return memo[n]
+
+
+# TLE
+class Solution2:
+ def tribonacci(self, n: int) -> int:
+ if n <= 1:
+ return n
+ elif n == 2:
+ return 1
+ return self.tribonacci(n-1) + self.tribonacci(n-2) + self.tribonacci(n-3)