LeetCode 535. Encode and Decode TinyURL

原题链接在这里：https://leetcode.com/problems/encode-and-decode-tinyurl/

题目：

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.

题解：

数字count其实就是base不同的short url.

Time Complexity: encode, O(count). count每次除以62. decode, O(n). n是short url的长度.

Space: O(m). 连个映射的大小.

AC Java:

public class Codec {
    HashMap<String, Integer> lToS = new HashMap<>();
    HashMap<Integer, String> sToL = new HashMap<>();
    int count = 1;
    String elements = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    String head = "http://tinyurl.com/";
    
    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        if(lToS.containsKey(longUrl)){
            return head + convertToBase62(lToS.get(longUrl));
        }
        
        String shortURL = convertToBase62(count);
        lToS.put(longUrl, count);
        sToL.put(count, longUrl);
        count++;
        return head + shortURL;
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
        int n = convertToBase10(shortUrl.substring(head.length()));
        return sToL.get(n);
    }
    
    private String convertToBase62(int n){
        StringBuilder sb = new StringBuilder();
        while(n > 0){
            sb.insert(0, elements.charAt(n % 62));
            n /= 62;
        }
        
        return sb.toString();
    }
    
    private int convertToBase10(String s){
        int res = 0;
        for(char c : s.toCharArray()){
            res = res * 62 + getInt(c);
        }
        
        return res;
    }
    
    private int getInt(char c){
        if(c >= '0' && c <= '9'){
            return c - '0';
        }else if(c >= 'a' && c <= 'z'){
            return c - 'a' + 10;
        }else if(c >= 'A' && c <= 'Z'){
            return c - 'A' + 36;
        }
          
        return -1;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.decode(codec.encode(url));

跟上Design TinyURL.