navigator

// 实现一个浏览器的前进、后退与前进网址的功能


current -> a

backup -> c, b


class Navigator {

  backup = [];
  current = [];


  forward(): string {
    // get from backward stack and push to current stack
    const result = this.backup.pop();

    if(!result) {
      return result 
    }

    current.push(result);

    return this.current.length === 0 ? "" : this.current[this.current.length - 1];
  }

  backward(): string {
    // get from current stack and push to backward stack
    const result = current.pop();
    this.backup.push(result);

    return this.current.length === 0 ? "" : this.current[this.current.length - 1];

  }

  visit(url: string):void {

    current.push(url);
  }

  refresh():void {

  }
}


const navigator = new Navigator();

navigator.visit('https://www.google.com'); // current = ['https://www.google.com'], backup = []
navigator.visit('https://www.youtube.com'); // current = ['https://www.google.com', 'https://www.youtube.com'], backup = []

expect(navigator.backward()).toBe('https://www.google.com'); // current = ['https://www.google.com'], backup = ['https://www.youtube.com']
expect(navigator.forward()).toBe('https://www.youtube.com'); // current = ['https://www.google.com', 'https://www.youtube.com'], backup = []
navigator.backward(); // current = ['https://www.google.com'], backup = ['https://www.youtube.com']
navigator.backward(); // current = [], backup = ['https://www.youtube.com', 'https://www.google.com']
navigator.visit('https://www.facebook.com'); // current = ['https://www.facebook.com'], backup = ['https://www.youtube.com', 'https://www.google.com']
expect(navigator.backword()).toBe(''); // current = [], backup = ['https://www.youtube.com', 'https://www.google.com', 'https://www.facebook.com']