$ cat ./coding/1137-n-th-tribonacci-number.md

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1137. N-th Tribonacci Number

自頂向下

class Solution:
    def tribonacci(self, n: int) -> int:
        
        memo = defaultdict(int)
        memo[0] = 0
        memo[1] = 1
        memo[2] = 1
        def helper(n):
            if n in memo:
                return memo[n]
            
            memo[n] = helper(n - 1) + helper(n - 2) + helper(n - 3)
            return memo[n]
        return helper(n)

自底向上

class Solution:
    def tribonacci(self, n: int) -> int:
        
        if n == 0:
            return 0
        elif n <= 2:
            return 1
        
        a = 0
        b = 1
        c = 1
        
        n -= 2
        
        while n > 0:
            a, b, c = b, c, a + b + c
            n -= 1
        
        return c

$ ls ./coding/ | grep -v 1137-n-th-tribonacci-number