这题其实是很常见的一个开发场景，短地址的开发。我这里只是分享一种md5的方式，还有其他的生成字符串比较短的hash方式。

import java.security.MessageDigest;import java.security.NoSuchAlgorithmException;import java.util.HashMap;public class Codec {	private HashMap
     
       map = new HashMap<>();	// Encodes a URL to a shortened URL.	public String encode(String longUrl) {		String md5 = crypt(longUrl);		String url = "http://tinyurl.com/" + md5;		map.put(url, longUrl);		return url;	}	// Decodes a shortened URL to its original URL.	public String decode(String shortUrl) {		return map.get(shortUrl);	}	public String crypt(String str) {		if (str == null || str.length() == 0) {			throw new IllegalArgumentException("String to encript cannot be null or zero length");		}		StringBuffer hexString = new StringBuffer();		try {			MessageDigest md = MessageDigest.getInstance("MD5");			md.update(str.getBytes());			byte[] hash = md.digest();			for (int i = 0; i < hash.length; i++) {				if ((0xff & hash[i]) < 0x10) {					hexString.append("0" + Integer.toHexString((0xFF & hash[i])));				} else {					hexString.append(Integer.toHexString(0xFF & hash[i]));				}			}		} catch (NoSuchAlgorithmException e) {			e.printStackTrace();		}		return hexString.toString();	}		public static void main(String[] args) {		String url = "https://leetcode.com/problems/design-tinyurl";		Codec codec = new Codec();		System.out.println(codec.decode(codec.encode(url)));	}}
     

　　

import java.util.LinkedList;import java.util.List;class Solution {    public List
     
       selfDividingNumbers(int left, int right) {		  List
      
        ret = new LinkedList<>();		  for(int i=left;i<=right;i++) {			  if(isSelfDividing(i)) {				  ret.add(i);			  }		  }		  return ret;	    }	  	   public boolean isSelfDividing(int n) {		  char[] chars = (n+"").toCharArray();		  for(char c :chars) {			  int cInt = c-'0';			  if(cInt ==0 ) {				  return false;			  }			  if(n%cInt!=0) {				  return false;			  }		  }		  return true;		  	  }}
      
     

　　

public class Solution507 {	   public boolean checkPerfectNumber(int num) {		  		   int sum = 1;		   for(int i=2;i<=Math.sqrt(num);i++) {			   int num1,num2=0;			   if(num%i==0) {				   num1=i;				   num2=num/num1;				   if(num1!=num2) {					   sum+=num1+num2;				   }else {					   sum = num1;				   }			   }			   		   }		   if (num ==sum && num!=1) {			   return true;		   }else {			   return false;		   }	        	    }		   	public static void main(String[] args) {		// TODO Auto-generated method stub		Solution507 s = new Solution507();		System.err.println(s.checkPerfectNumber(1));		System.err.println(s.checkPerfectNumber(2));		System.err.println(s.checkPerfectNumber(4));		System.err.println(s.checkPerfectNumber(28));	}}

　　

class Solution { 	  public boolean judgeSquareSum(int c) {		 for(int i=0;i<=Math.sqrt(c);i++) {			 int lastNums = c -i*i;			 int last = (int) Math.sqrt(lastNums) ;			 if(last*last == lastNums) {				 return true;			 }					 }		 return false;	        	    }}