>大问题拆解为小问题，逐级向下拆解

## 用循环实现斐波那契数列

```javascript
const Fibonacci_Cycle = (n: number): number => {
	if (n <= 0) return 0;
	if (n === 1) return n;

	let n1 = -1;
	let n2 = -1;
	let n3 = 0;

	for (let index = 0; index <= n; index++) {
		if (index === 0) {
			n1 = 0;
			n2 = 0;
			continue;
		}
		if (index === 1) {
			n1 = 0;
			n2 = 1;
			continue;
		}

		n3 = n1 + n2;
		n1 = n2;
		n2 = n3;
	}
	
	return n3;
};
```