1137. N-th Tribonacci Number

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.

这题是一题简单的动态规划，后面的结果可以依赖前面的结果来解出。

class NthTribonacciNumber : public Solution {
public:
    void Exec() {

    }
    int tribonacci(int n) {
        int prev[3] = {0, 1, 1};
        if (n < 3) {
            return prev[n];
        }

        for (int i = 3; i <= n; i++) {
            int sum = prev[2] + prev[1] + prev[0];
            std::swap(prev[0], prev[1]);
            std::swap(prev[1], prev[2]);
            prev[2] = sum;
        }
        return prev[2];
    }
};