feat: tribonacci

Author: liguanliang1

leetcode/dynamic-programing/fib.js

@@ -19,3 +19,25 @@ function fib(n) {
 	return result
 }
 
+// fib(n+3) = fib(n) + fib(n+1) + fib(n+2);
+
+function tribonacci(n) {
+	let fibn_1 = 0
+	let fibn_2 = 1
+	let fibn_3 = 1
+	if (n === 0) { return fibn_1 }
+	if (n === 1) { return fibn_2 }
+	if (n === 2) { return fibn_3 }
+
+	let i = 3
+	let result = fibn_1 + fibn_2 + fibn_3;
+	while (i <= n) {
+		result = fibn_1 + fibn_2 + fibn_3;
+		fibn_1 = fibn_2;
+		fibn_2 = fibn_3;
+		fibn_3 = result;
+		i++
+	}
+
+	return result
+}