第 N 个泰波那契数

/*
 * Problem: 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 an integer n, return the nth Tribonacci number Tn.
 * 
 * Example 1:
 * Input: n = 4
 * Output: 4
 * Explanation:
 * T3 = 0 + 1 + 1 = 2
 * T4 = 1 + 1 + 2 = 4
 * 
 * Example 2:
 * Input: n = 25
 * Output: 1389537
 * 
 * Constraints:
 * 0 <= n <= 37
 * The answer is guaranteed to be a 32-bit integer, i.e., answer <= 2^31 - 1.
 */

import java.util.stream.IntStream;

public class TribonacciStream {
    public int tribonacci(int n) {
        if (n == 0) return 0;
        if (n == 1 || n == 2) return 1;
        int[] trib = {0, 1, 1};
        IntStream.range(3, n + 1).forEach(i -> {
            int next = trib[0] + trib[1] + trib[2];
            trib[0] = trib[1];
            trib[1] = trib[2];
            trib[2] = next;
        });
        return trib[2];
    }

    public static void main(String[] args) {
        TribonacciStream tribonacci = new TribonacciStream();
        System.out.println(tribonacci.tribonacci(4));  // Output: 4
        System.out.println(tribonacci.tribonacci(25)); // Output: 1389537
    }
}