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###### tags: `Leetcode`

# 1137. N-th Tribonacci Number

###### Link : https://leetcode.com/problems/n-th-tribonacci-number/description/

## 題目
回傳Tribonacci Number的第n項
## 程式碼 I
vector version

class Solution {
public:
    int tribonacci(int n) {
        if(n == 0 || n == 1) return n;
        else if(n == 2) return 1;
        vector<int> tri(n+1);
        int last = 3;
        tri[0] = 0, tri[1] = 1, tri[2] = 1;
        while(last <= n){
            tri[last] = tri[last - 3] + tri[last - 2] + tri[last-1];
            ++last;
        }
        return tri[n];
    }
};

## 程式碼 II
陣列版本 faster than vector

class Solution {
public:
    int tribonacci(int n) {
        if(n == 0 || n == 1) return n;
        else if(n == 2) return 1;
        int tri[n+1], last = 3;
        tri[0] = 0, tri[1] = 1, tri[2] = 1;
        while(last <= n){
            tri[last] = tri[last - 3] + tri[last - 2] + tri[last-1];
            ++last;
        }
        return tri[n];
    }
};

## Date
### 2023/1/30