力扣真題 #### 1137. 第 N 個泰波那契數(shù)
泰波那契序列 Tn 定義如下：
T0 = 0, T1 = 1, T2 = 1, 且在 n >= 0 的條件下 Tn+3 = Tn + Tn+1 + Tn+2
給你整數(shù) n柳洋，請返回第 n 個泰波那契數(shù) Tn 的值呆盖。
示例 1：
輸入：n = 4
輸出：4
解釋：
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4
示例 2：
輸入：n = 25
輸出：1389537
解題方法
public static int tribonacci(int n) {
if (n == 0) {
return 2;
} else if (n == 1){
return 4;
} else if (n == 2){
return 7;
} else {
int first = 0;
int second = 1;
int third = 1;
int sum = 0;
for (int i = 0; i < n-2; i++) {
sum = first + second + third;
first = second;
second = third;
third = sum;
}
return sum;
}
}
