1137.第-n-个泰波那契数.py

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# @lc app=leetcode.cn id=1137 lang=python3
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# [1137] 第 N 个泰波那契数
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# https://leetcode.cn/problems/n-th-tribonacci-number/description/
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# algorithms
# Easy (60.81%)
# Likes:    203
# Dislikes: 0
# Total Accepted:    124.3K
# Total Submissions: 204.4K
# Testcase Example:  '4'
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# 泰波那契序列 Tn 定义如下： 
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# T0 = 0, T1 = 1, T2 = 1, 且在 n >= 0 的条件下 Tn+3 = Tn + Tn+1 + Tn+2
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# 给你整数 n，请返回第 n 个泰波那契数 Tn 的值。
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# 示例 1：
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# 输入：n = 4
# 输出：4
# 解释：
# T_3 = 0 + 1 + 1 = 2
# T_4 = 1 + 1 + 2 = 4
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# 示例 2：
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# 输入：n = 25
# 输出：1389537
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# 提示：
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# 0 <= n <= 37
# 答案保证是一个 32 位整数，即 answer <= 2^31 - 1。
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# @lc code=start
class Solution:
    def tribonacci(self, n: int) -> int:
        if n == 0:
            return 0
        if n in [1,2 ]:
            return 1
        n = n-2
        a,b,c = 0,1,1
        while n :
            a,b,c = b,c,a + b + c
            n-=1
        return c
# @lc code=end