Encode and Decode TinyURL

TinyURL is a URL shortening service where you enter a URL such as 

https://leetcode.com/problems/design-tinyurl

 and it returns a short URL such as 

http://tinyurl.com/4e9iAk

.Design the 

encode

 and 

decode

 methods for the TinyURL service. There is no restriction on how your encode/decode algorithm should work. You just need to ensure that a URL can be encoded to a tiny URL and the tiny URL can be decoded to the original URL.class Solution {
public:

Solution() {
dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
short2long.clear();
long2short.clear();
srand(time(NULL));
}

// Encodes a URL to a shortened URL.
string encode(string longUrl) {
if(long2short.count(longUrl))
{
return "http://tinyurl.com/"+long2short[longUrl];
}
int idx = 0;
string randStr;
for(int i=0; i<6; i++) randStr.push_back(dict[rand()%62]);
while(short2long.count(randStr))
{
randStr[idx] = dict[rand()%62];
idx = (idx+1)%6;
}
long2short[longUrl] = randStr;
short2long[randStr] = longUrl;
return "http://tinyurl.com/"+long2short[longUrl];
}

// Decodes a shortened URL to its original URL.
string decode(string shortUrl) {
string randStr = shortUrl.substr(shortUrl.find_last_of("/") + 1);
return short2long.count(randStr) ? short2long[randStr] : shortUrl;
}

private:
unordered_map<string, string> short2long, long2short;
string dict;
};

// Your Solution object will be instantiated and called as such:
// Solution solution;
// solution.decode(solution.encode(url));