Web Crawler solution and explanation

# """
# This is HtmlParser's API interface.
# You should not implement it, or speculate about its implementation
# """
# class HtmlParser(object):
#    def getUrls(self, url):
#        """
#        :type url: str
#        :rtype List[str]
#        """


class Solution:
    def crawl(self, startUrl: str, htmlParser: 'HtmlParser') -> List[str]:
        def host(url):
            url = url[7:]
            return url.split('/')[0]

        def dfs(url):
            if url in ans:
                return
            ans.add(url)
            for next in htmlParser.getUrls(url):
                if host(url) == host(next):
                    dfs(next)

        ans = set()
        dfs(startUrl)
        return list(ans)

/**
 * // This is the HtmlParser's API interface.
 * // You should not implement it, or speculate about its implementation
 * interface HtmlParser {
 *     public List<String> getUrls(String url) {}
 * }
 */

class Solution {
    private Set<String> ans;

    public List<String> crawl(String startUrl, HtmlParser htmlParser) {
        ans = new HashSet<>();
        dfs(startUrl, htmlParser);
        return new ArrayList<>(ans);
    }

    private void dfs(String url, HtmlParser htmlParser) {
        if (ans.contains(url)) {
            return;
        }
        ans.add(url);
        for (String next : htmlParser.getUrls(url)) {
            if (host(next).equals(host(url))) {
                dfs(next, htmlParser);
            }
        }
    }

    private String host(String url) {
        url = url.substring(7);
        return url.split("/")[0];
    }
}

/**
 * // This is the HtmlParser's API interface.
 * // You should not implement it, or speculate about its implementation
 * class HtmlParser {
 *   public:
 *     vector<string> getUrls(string url);
 * };
 */

class Solution {
public:
    vector<string> ans;
    unordered_set<string> vis;

    vector<string> crawl(string startUrl, HtmlParser htmlParser) {
        dfs(startUrl, htmlParser);
        return ans;
    }

    void dfs(string& url, HtmlParser& htmlParser) {
        if (vis.count(url)) return;
        vis.insert(url);
        ans.push_back(url);
        for (string next : htmlParser.getUrls(url))
            if (host(url) == host(next))
                dfs(next, htmlParser);
    }

    string host(string url) {
        int i = 7;
        string res;
        for (; i < url.size(); ++i) {
            if (url[i] == '/') break;
            res += url[i];
        }
        return res;
    }
};

/**
 * // This is HtmlParser's API interface.
 * // You should not implement it, or speculate about its implementation
 * type HtmlParser struct {
 *     func GetUrls(url string) []string {}
 * }
 */

func crawl(startUrl string, htmlParser HtmlParser) []string {
	var ans []string
	vis := make(map[string]bool)
	var dfs func(url string)
	host := func(url string) string {
		return strings.Split(url[7:], "/")[0]
	}
	dfs = func(url string) {
		if vis[url] {
			return
		}
		vis[url] = true
		ans = append(ans, url)
		for _, next := range htmlParser.GetUrls(url) {
			if host(next) == host(url) {
				dfs(next)
			}
		}
	}
	dfs(startUrl)
	return ans
}