## 用两个栈实现一个队列

```javascript
class StackQueue {
	private stack1: number[] = [];
	private stack2: number[] = [];
	
	add(value: number) {
		this.stack1.push(value);
	}
	
	popup() {
		if (!this.stack1.length) return;
		while (this.stack1.length) {
			const stackValue = this.stack1.pop();
			this.stack2.push(stackValue!);
		}
	
		const popValue = this.stack2.pop();
		
		while (this.stack2.length) {
			const stackValue = this.stack2.pop();
			this.stack1.push(stackValue!);
		}
		
		return popValue;
	}
	
	get length() {
		return this.stack1.length;
	}
}
```