1137. N-th Tribonacci Number*
https://leetcode.com/problems/n-th-tribonacci-number/
題目描述
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
.
C++ 實現 1
使用迭代的方法, 思路很直接.
class Solution {
public:
int tribonacci(int n) {
if (n == 0) return 0;
if (n <= 2) return 1;
int a = 0, b = 1, c = 1;
n -= 2;
while (n --) {
int sum = a + b + c;
a = b;
b = c;
c = sum;
}
return c;
}
};